Jack Straw is putting the theory of survival of the fittest to the test. At lunchtime, as he does most days, he made his way to the parliamentary gym, where he took part in an hour of ‘bodyblasting’ – aerobics with weights. He’s 67, but other than a head of hair which has turned entirely white, age has not weathered him. Straw is not just the last man standing, the title of his memoirs, but the last man bodyblasting, running, spinning and weight-lifting.

But even survivors can’t survive forever.

As he edges towards 35 years as an MP, Straw has decided to retire. The great survivor recognised he was up against an immovable foe: time’s winged chariot.

“I’ll be touching 69 by the 2015 election, so I’d be committing myself to my mid-70s if I was to stay on. Much as I love the job, and this place and the constituency, I just was not certain that I’d be able to give the twin commitment of the constituency and the Commons the energy that is needed.”

At the start of 2013 Straw began to mull over the idea of retirement. By September, after talking to his wife Alice and members of his local party, he had decided that it was “time for me to go.”

Even the most rigorous work out schedule, it seems, could not defeat the advancing years.

“I don’t feel old. I feel ridiculously young. I don’t feel my age but it’s just a fact of life. I’m in my late 60s. You start to have friends who become very ill. Some pass away. Aging is a process you can’t do anything about. I don’t feel old in my head.”

But he must feel old when he looks at a party run by men and women who were special advisers to the ministers of the early New Labour years? Straw smiles. “Many of them are the same age as my children, in some cases younger. But that’s life…”

Straw himself was just 28 when he began his Westminster career, an early example of the professional politician. Other than a spell at the Bar, his career has been spent entirely in politics: from leading the National Union of Students to working as a special advisor for Labour minister Barbara Castle and then into Parliament. Castle saw something that others would soon become familiar with, praising Straw’s “guile and low cunning” – a tribute of sorts which will forever follow him.

“Partly I gave it a wider audience”, Straw replies, when asked if he minds the durability of the character reference. “It’s one aspect of any successful politician, I reckon, but hopefully it will not be my only epitaph.”

Straw is speaking in his small Westminster office, which he moved into in 2010. The new home probably took some getting used to. He spent the entire 13 years of New Labour’s time in government in the cabinet – one of just three MPs, along with Gordon Brown and Alistair Darling, to do so – and so enjoyed the departmental and Commons offices that came with his quartet of ministerial positions: home secretary, foreign secretary, leader of the Commons and justice secretary.

It’s a CV, along with a slog through the party’s 18 long years in opposition, which demonstrate his instinct for political survival.

On a personal level, Straw survived a broken home after his father walked out on the family, the death of his first child, a divorce, and a battle with depression. As a child he passed the eleven plus and made his way to grammar school and onto Leeds University, but for a time, he wrote, he felt gripped by “imposter syndrome”, a feeling that all that he had achieved would be taken away. When did that leave him? “I guess I finally nailed that ghost sometime during my period as home secretary," he replies of a post which he held for New Labour’s first term in office, skilfully sidestepping the pitfalls of a department that has played host to the end of many a ministerial career – including that of his immediate successors, David Blunkett and Charles Clarke. The one-time long haired leader of the NUS may have surprised many of Labour’s libertarian left with his tough approach to crime, but the inquiry he ordered into the murder of the black teenager Stephen Lawrence, and the full implementation of the Macpherson report which followed, is seen as a landmark achievement of the Blair government. Straw also highlights a reduction in crime, “most of which started with the [1998] Crime and Disorder Act”, and, more generally, Labour’s record on human rights and freedom of information.

Not everyone agrees on the last point. In fact, the most vocal opponent is the prime minister who approved bringing in FOI: Tony Blair says he now “quakes at the imbecility” of it. Straw shakes his head.

“We needed a FOI act”, he insists, though he admits the legislation was, and remains, flawed. “Its gestation was just horrific. No staff work was done on it in opposition. The FOI campaign basically led the policy in opposition and then in the early stages of government. Their propositions went too far and when Tony realised this he passed this smoking package to me… I pulled back on it a lot but by then we’d got this first white paper which was the most liberal FOI policy in the world, and probably Mars as well. We ended up with an act which is not satisfactory, not crafted well, and doesn’t properly protect the kind of thinking space that governments needed. But people are learning to live with it.”

Whatever the niggling doubts over FOI, Straw’s record as foreign secretary is far, far more controversial. Asked to pick out his highlights at the FCO, Straw talks about managing to force an EU-wide agreement on Turkish accession – “that’s’ gone off the boil, but it’s important” – and his role in the E3 meetings between the UK, France, and Germany which “stopped any idea that there should be military attacks and therefore a war with Iran which many of the neo-cons in the US were getting ready for." Ask anyone else about Jack Straw's legacy at the Foreign Office, and the answer will settle on just one word: Iraq.

Straw had his doubts about military action, but eventually came round to the arguments for sending British troops to Iraq. But on March 18th 2003, the foreign secretary told the House of Commons why it should vote for war. “There was an acute decision, a binary decision. I made my choice. I know that if I decided to make a contrary choice at that stage there would not have been a majority. That’s not conceit, that’s a fact of life.”

An entire shelf in his office heaves with notes and folders on Iraq, evidence which Straw poured through ahead of his 2010 appearance before the Chilcot inquiry into the invasion of Iraq. He admitted then that he is still haunted by his choice. Is he today? “Yes. What troubles me particularly is the aftermath of military action. We felt out of control over that. I am happy to explain and justify my personal decision I made to support military action. That was a decision in our control, not least in mine. The frustration about the aftermath… still makes me angry.”

Straw worked well with the George W. Bush’s first secretary of state Colin Powell, and struck up a close rapport with Powell’s successor Condoleeza Rice, who famously took in a trip to Straw’s Blackburn constituency. Powell had ‘a good plan’ for post-invasion Iraq, Straw says, complaining that it was all “completely undermined by Donald Rumsfeld and Dick Cheney” and their “disastrous decisions.”

Yet as the scenes of chaos and carnage were relayed daily from Iraq, Straw stayed loyal to Blair. “I saw no point in walking way. I had a responsibility,” Straw makes clear. “I was one of the people who had contributed to those circumstances, whether advertently or inadvertently.”

His reward for his loyalty was not what he expected: in 2006 Blair removed Straw from the global stage and made him leader of the Commons. It stung.

“Yes it did. I was upset about it. It hurt. I thought it was unjust,” Straw recalls of a demotion which prompted surprise in the media and later regret from Blair. Straw briefly considered resignation, but again that instinct to survive kicked in.

Three years on from the Iraq invasion Straw had become a less popular figure in the US. Perhaps with the wounds of Iraq still raw, Straw upset the White House by insisting that it was “inconceivable” that the UK would support military action in Iran and, rather less diplomatically, describing the idea of a US nuclear strike as “completely nuts.” Could Bush have leant on Blair to remove his foreign secretary?

“People say that,” Straw replies. “The neo cons, people like John Boulton, were never terribly keen on me, but I’ve no idea what observations were made through the back door. Tony and I were getting to a different place on handling Iran and handling Hamas, and Tony certainly felt disconcerted when I said that it was inconceivable that the UK would go to war with Iran and described a nuclear attack as nuts. As events have proved, we haven’t been involved in military action in Iran – it’s inconceivable now: there’s no way a British House of Commons would approve any kind of military action in Iran – and a nuclear strike remains nuts. Anyway…”

Intriguingly, Straw also suggests that had he stayed at the Foreign Office, and closer to the centre of power, he might have prolonged Blair’s premiership.

“It detached me from Tony: if I’d carried on being foreign secretary I’d have almost saved him from himself,” he argues, pointing to Blair's refusal to distance himself publicly from the White House's support for Israel's 2006 attacks on Hezbollah fighters in Lebanon. “He got himself totally out on a limb over the Israelis’ incursion into Lebanon,” Straw continues. “The Israelis were doing unspeakable things and acting in a completely disproportionate way and he couldn’t find the words to say this. We argued about it in cabinet, but if I’d been his foreign secretary, with him hour by hour, I think I’d have been able to persuade him to take the view he finally took, but it was too late then. It gave the opportunity to people who had always been opposed to him to propose that he should go.”

Within a year, Blair had resigned. The decision left Straw feeling “sad, really sad, but he had created the circumstances where that was inevitable.”

He says “all things being equal” he would rather Blair had stayed in office, but he still managed to move, apparently with minimum fuss, into the camp of Blair’s successor, Gordon Brown. Having run Blair’s leadership campaign back in 1994, he was now running Brown’s in 2007. It was an apparently seamless transition from one rival New Labour camp to the other, and saw Straw accused of swapping allegiances – and shelving principle – in the name of his own career survival. “I know. But it wasn’t that seamless,” Straw breezily replies, before justifying his decision. “By the time of the leadership contest Gordon was almost certainly going to become leader. He asked me to become his campaign manager so I said yes. It seemed to be a better answer than saying no.”

But couldn’t Straw have kept the Blairite flag flying by at least challenging the favourite? “There was no point me standing in 2007. Gordon had the numbers”, he replies. “It would have taken a huge effort for no particular purpose.”

Straw insists he has no regrets about the decision to back Brown, but admits that he “had not anticipated the difficulty he had in making decisions, which had not been exposed when he was chancellor.” Straw cites Brown’s dithering over calling a snap election soon after he moved into Number 10 as an example. “My view was that he shouldn’t go to the polls in 2007 and I said that to him, but the worst thing was that he let the thing drift for two weeks then went to Afghanistan in the middle of the Conservative conference which was crass in the extreme.”

In 2009, with Brown under increasing pressure following a string of Cabinet resignations, speculation began to mount that Straw could mount a leadership challenge in which the numbers actually would have added up. Again, however, he resisted.

“I suppose [it was] a nervousness about the old… ‘he who wields the sword never wears the crown,’” Straw replies when asked what held him back. “Also, whether we’d be left with the wreckage of the party: I think that applies to the others involved: David Miliband, Alan Johnson,” he adds, pointing to the other prospective leadership candidates. “We can be fairly criticised for being ready to wound but afraid to strike. But it’s a fact about me: I’m loyal to the party. I was worried about that and it wasn’t clear that the numbers were there, and even if one won….”

Straw tails off. Surely Labour’s 2010 general election defeat left him cursing his caution? “Not particularly. In fact I think we did rather well in 2010 given the circumstances.”

The moment, or moments, had passed, and Straw returned to the same opposition benches he had sat on from 1979 to 1997, with Ed Miliband winning the contest for the Labour leadership. So is the present leader a more ruthless or ambitious politician than Straw ever was?

“He’s probably more ambitious, yeah,” Straw replies. “It depends what’s inside you. You have to be very hungry to become party leader and I’ve never been that bothered about it. If someone walked into the room and said ‘Mr Straw, you can be prime minister, sign here’, I’d sign and I would not have been a bad prime minister. I was never very keen on being leader of the opposition.”

Nor was he very keen on a shadow cabinet post in Ed’s team. “It wasn’t that he didn’t ask me, I made it clear to him, as I would have done with David Miliband, that I didn’t want to go on the opposition front bench again. I wanted to do other things.”

So free at last from the ministerial red boxes and diary demands of a frontbencher, Straw has settled into the role of backbench grandee, speaking out and writing on both international and domestic issues.

This year is likely to see Iraq return to the front pages again as the Chilcot report is finally published. “I’d like it to come out, I’d like to know what he’s going to say rather than not”, insists Straw. “It doesn’t keep me awake at night, I just think it would be important to have the report sooner rather than later.”

Will Chilcot bring closure? Straw isn’t sure. “I don’t know. People will go on arguing about the decisions. It was one of the most significant foreign policy decisions made, certainly since Suez, and it continues to have repercussions.”

After 2015, when he departs the Commons for the final time, Straw says he will see more of his grandchildren – his first two were born within two days of each other last year – and hopes to travel abroad with his wife.

There’s a chance that the Straw name will continue in the House of Commons – Jack’s son Will is the Labour candidate for Rossendale and Darwen, though Straw Snr. passes when asked to comment on the possible creation of a Straw dynasty – but don’t expect a quiet retirement from Straw the Elder: he will continue to follow policy developments, and “certainly won’t be spending my time with the proverbial pipe and slippers.”

Blackburn, he says, is “part of my soul”, and he intends to carry on doing what he can for the town, including in his role as governor of Blackburn College, while at a national level he confirms that he is braced to joining the battle over Britain’s future in Europe. “I was thinking about that this morning, funnily enough – if there was an EU referendum I would weigh in on the Yes side,” Straw reveals. “I was in the No campaign in 1975 and my view would have been the right decision at the time. [Now] it would damage Britain’s interests significantly if we were to leave. I get frustrated by the EU, but if forced to make a decision I would say yes. In the end, because people vote with their heads not their hearts, it will be won.”

It’s safe to predict that Straw won’t be travelling far to make himself heard: as a former holder of two great offices of state, expect Lord Straw to be sitting on the red benches of the House of Lords before long – even if, during his time as Commons leader, he floated plans to bring in an elected Chamber. “That’s true”, he shrugs. So would he go to the Lords? “Would I? Yes is the answer – as long it’s appointed, but we’ll see.”

The end may be approaching, but, as ever, Jack Straw has an eye for survival. That parliamentary gym membership won't be expiring anytime soon.