Alistair Darling told a Birmingham student radio station that he would not take a position on the shadow frontbench if Ed Miliband asked him.
And the former chancellor admitted that David Cameron's recent speech on the Scottish question was "the most intelligent" from a Conservative leader in 30 years.
In an interview with Burn FM, Darling said he did not want to stand as Labour leader after Ed Miliband.
"I'm on the backbenches now," he said. "I was in government for 13 years. I'm quite happy where I am. I have said that I intend to stand in a lot of this Parliament, being involved in the pro-union campaign to keep Scotland in the United Kingdom. So that's what I'll be spending my time on. That's all I'm focused on."
Darling revealed he would not take up a frontbench role if asked by Ed Miliband, saying: "Well, not at the moment because I am enjoying the backbenches. I can look at things that interest me like the economy for example.
"I can say as much, if not more, where I am. I don't need to be on the frontbench if I want to get on the television to say something. I care passionately what happens to Scotland and that is what I intend to spend my time doing. "
And he showed a rare glimpse of unity between himself and the Conservatives over the question of Scottish independence.
"What I would say about David Cameron – and I disagree with him on a whole host of things – but he was in Scotland recently and he probably made the most intelligent speech on the question of Scotland's future that any Tory leader has in the last 30 years," said Darling.
"He came at it from the point of view that, 'Look we in Scotland as Tories have very little support. We recognise that Scotland has changed and it has changed dramatically in the last 25-30 years.' And he said, 'It's up to you.'
"He happened to think, as I happen to think, that we are better off in the union than going our separate ways. I think that making the point that the referendum has to be legally binding, that it has to be the Electoral Commission, those are both things that Alex Salmond wanted to avoid. That's a thoroughly good thing."