Mark Serwotka does not like the idea of Tony Blair's second political wave.

"Any idea of a return to politics for Tony Blair is appalling," he tells me.

"He's done his damage and he's best doing what he does now."

We have been speaking about the PCS's decision to field candidates at future elections – potentially even standing anti-austerity figures against the Labour Party (you can read about that here).

The general secretary suggests the move is a response to the damage New Labour did to the party – and Blair was at the heart of that.

"Some people hail him as somebody of historic importance because he won Labour three elections, that's true," he says.

"My own view is that he'll go down as one of the worst Labour leaders ever for the simple reason that he had the opportunity – having had 18 years of the Tories – to really shake things up in Britain and he chose not to do it.

"He chose to do the opposite and drag the Labour Party into the centre, to the right. When history judges that in time to come, people will view that as shocking…

"[It] brings shame on the Labour movement in my view."

Serwotka is more flattering about current Labour leader Ed Miliband – but even he has serious faults.

How do you think Miliband is doing? I ask.

There's a long pause. "Put it like this: I think it's good that there have been occasions in the recent past where Labour has been much more assertive in opposition," he replies, pointing to Leveson and Barclays as examples.

"But then you have to counter-balance it with their absurd support for the public sector pay freeze, which in our view is economically stupid.

"It's the last thing we need. And yet they've come out and done it, alienating millions of people who might be Labour voters.

"And they won't pick up a single vote as a result of it.

"There is that inherent lack of clarity still about which way some of these people are going."

Serwotka's union is not affiliated to the Labour Party, and has often been critical.

But now his union's political fund represents a threat to the electoral fortunes of some Labour candidates.

It gives critical comments like these an edge - and puts additional pressure on the party.

That's exactly what Serwotka wants.

Tags: Ed Miliband, Mark Serwotka, Pcs, Tony Blair, Trade union funding