Gordon Brown has been urged to outline his "vision for the future of the UK" by former Lord Chancellor, and leading Blairite, Lord Falconer.
Lord Falconer has been a long-standing staunch ally of Tony Blair
He said if Labour governed only on the basis of past experience, there could be "drift", adding: "Renewal does not come from change of leadership alone."
One opinion poll gives the Tories a seven-point lead over Labour.
Cabinet minister John Hutton said Mr Brown had "vision" and would "rise above all this brouhaha".
But shadow foreign secretary William Hague said the "tectonic plates" in politics had "shifted" in favour of the Conservatives.
'Past not future'
Lord Falconer, who left the Cabinet when Tony Blair stood down as prime minister in June, told the Sunday Times: "Making clear our vision is the challenge for the Labour Party now.
"Because if you rely on experience and our ability to handle crises and do not set out, in the coming months, our vision for the future of the UK... we will be offering drift not leadership, and the past not the future."
BBC political correspondent Gary O'Donoghue said former Blairite cabinet ministers had largely kept their counsel about Mr Brown until now, making this "a significant moment, coming at the end of a difficult week for the prime minister".
Olympics Minister Tessa Jowell said suggestions that supporters of Mr Blair were preparing an onslaught against Mr Brown were "absolutely and categorically untrue".
She told Sky News it was "nothing short of wicked" to suggest that Tony Blair had done any more than "become principally and primarily engaged in trying to achieve peace in the Middle East".
The former prime minister "wants Gordon Brown to be a successful New Labour prime minister. He wants a successful Labour government," she said.
'Tectonic plates shifting'
Mr Hutton, the industry secretary, told BBC One's Andrew Marr show that politics was a "rough trade sometimes".
He added: "What we have got to do is rise above all this brouhaha."
He did agree though that Labour had to show its "vision for the future", adding: "It's a vision thing. That's what people are looking for... We have a very clear vision.
Gordon Brown is still ahead in the leadership polls
"New Labour is going to remain in business as a political concern."
The Conservative's Mr Hague told ITV1's Sunday Edition: "We've got that huge advantage now in politics that we know what we stand for, and I don't think either of the other parties now know what they stand for in politics any more."
However, Europe minister Jim Murphy told Sky News: "Gordon Brown is a man of immense substance.
"I am very confident that in time, and over the next few months and longer, there are contrasts between Gordon's real substance and David Cameron's relatively lightweight, all spin, very little substance.
"And the population of the United Kingdom will be very clear who they want to be their leader and I'm certain it would be Gordon Brown."
An ICM opinion poll for The Sunday Telegraph puts the Tories on 43%, Labour on 36% and the Liberal Democrats on 14%.
The findings, obtained from a random sample of 1,010 adults phoned on 10 and 11 October, suggest the Conservative Party is now more popular than it has been at any other time in the last 15 years.
Two weeks ago, during the Labour Party conference, Labour held an 11-point lead over the Tories in some opinion polls.
But since then, Labour's popularity appears to have been hit by Tory initiatives on inheritance tax, rows over troop withdrawals from Iraq, and the speculation over an autumn election that never came.
The ICM poll does, however, show that Mr Brown is still seen by those surveyed as a stronger leader than Mr Cameron, by 52% to 32%.