Lord Mandelson today condemned Ed Miliband for being too quick to "criticise his own side".
The former business secretary also warned the Labour leader not to cede the centre ground to the Tories or he could lose the next election.
Speaking on ITV's Daybreak, Lord Mandelson said: "I like Ed Miliband. I know him fairly well, I support him as leader and, to be frank, whatever course he follows I'm going to be supporting him because I want to see a Labour government instead of the people we've got at the moment.
"I just think that sometimes he's a bit too quick to criticise his own side and the last Labour government.
"He's so quick to say he's not New Labour, he's not Tony Blair and the rest. Well, it's fine to be something different but you've also got to make clear what that something else is, and I don't think he's done so yet."
Lord Mandelson also suggested that Labour may have won the last year's election if Tony Blair had still been at the helm of the party rather than Gordon Brown.
Asked if Mr Blair could have won the election he replied: "I think it's a possibility that he would have done, but he thought it was time to move on and he was fairly well pushed out of government in 2007.
"If he had stayed on I think he would have been more clearly New Labour, very clearly occupying that centre ground where most of the voters are and where elections are won in our country.
"And what we are seeing now is Cameron and his lot trying desperately hard to be like New Labour, to be centrist, to be moderate, to marginalise their right wing, although I think a lot of their policies are tilting in a different direction."
He added: "If we continue to cede that ground to Cameron and the Tories then we are going to be in trouble at the next election."