David Miliband has been touted as a possible successor to Gordon Brown
Foreign Secretary David Miliband has called on Labour colleagues to find the confidence to make their case afresh.
Writing in the Guardian, he says New Labour won three elections by offering real change, and must do so again.
But Mr Miliband, who is seen as a potential successor to Gordon Brown, avoids mentioning the PM at all.
His comments will be seen by many people as setting out his stall should speculation about a new Labour leader continue over the summer.
However, ex-cabinet minister Peter Mandelson, now Britain's EU Trade Commissioner, said: "It would be odd if the government didn't have people who were in a position to make their mark, to make their contribution, in the way that David is doing."
'Vision for the future'
He said it would be more surprising if "a leading figure in the government, like the foreign secretary, didn't address himself to the challenges the government is facing".
The comments come as Mr Brown faces increasing pressure following Labour's Glasgow East by-election defeat to the SNP.
In his article, Mr Miliband warns: "The odds are against us, no question. But I still believe we can win the next election."
But he says: "In the aftermath of Labour's third successive defeat at the 1959 election, a famous pamphlet asked the question: 'Must Labour lose?'
"Today, the temptation is similar fatalism. We must not yield to it.
"We need to remember that there is little real sense among the public - or even among Tory MPs - of what the Conservatives stand for, or what they would do in power."
He continues: "I agree with Jack Straw that we don't need a summer of introspection.
"The starting point is not debating personalities but winning the argument about our record, our vision for the future and how we achieve it."
He lists Labour's successes and failures, and dismisses Conservative leader David Cameron's claims of a "broken society".
Mr Miliband notes that crime is down, lone parent employment and school standards are up, and that there are fewer asylum seekers.
However, he says with hindsight the government "should have got on with reforming the NHS sooner", planned for how to win the peace in Iraq and devolved more power away from Whitehall.
He says Mr Cameron is a "likeable" but "empty" politician of the status quo.
Setting out his vision for the future, Mr Miliband says "times demand a radical new phase".
He ends: "New Labour won three elections by offering real change, not just in policy but in the way we do politics. We must do so again.
"So let's stop feeling sorry for ourselves, enjoy a break and then find the confidence to make our case afresh."
Ex-Europe minister Denis MacShane welcomed the foreign secretary's comments, adding that "ministers and Labour MPs should follow Miliband's leadership" in turning their fire on the Tories.
He described Mr Miliband's article as "a sharp and welcome contrast to the self-indulgent, defeatist briefings by ministers in the last few days".
"David's succinct and forward-looking case for Labour, combined with his deconstruction of the emptiness and contradictions of Cameronism, is to be welcomed," he said.