Speculation about Gordon Brown's future has increased recently
A group of ex-Labour ministers are to set out their own policy agenda to fill what they see as a "vacuum" at the heart of government.
The unnamed former ministers told the BBC their list of four or five initiatives would distinguish Labour from David Cameron's Conservatives.
The move, expected within weeks, will increase the pressure on Gordon Brown.
Three cabinet ministers have attempted to damp down speculation about his leadership by rallying to his defence.
The BBC's Iain Watson reports that one of the group of former ministers stated: "Gordon Brown's back is against the wall."
Another said of the pressure on the prime minister: "He's become more obsessed with the leadership than with running the country."
But, our correspondent added, Mr Brown did have avenues available to fight back against criticism of his leadership, including a possible late summer cabinet shuffle and plans for a new economic policy.
The ex-ministers' policy move came after Alistair Darling, Harriet Harman and John Denham lined up on Sunday to declare their support for the prime minister.
Skills Secretary Mr Denham told BBC One's Andrew Marr programme that Mr Brown had a "profound understanding of what this country needs" and that part of cabinet's job is to spread the message that the right man is already in the role.
Gordon Brown is the right person for the job
Harriet Harman Deputy Labour leader
"Anything that gets in the way of all of us as a team putting that message across effectively is a distraction from what needs to be done, and is in danger of letting David Cameron win by default."
Deputy leader Harriet Harman told the News of the World that the party needed someone who had the ability to see Britain through the current economic downturn.
John Denham criticises leadership speculation
"Gordon Brown is the right person for the job," she said.
And Chancellor Alistair Darling praised the prime minister's "determination and strong purpose".
Despite the speculation that has raged over Mr Brown's future since Labour's recent by-election defeat in Glasgow East, Mr Denham said he believed the Labour party could win the next election.
The ministers' comments follow an article written by Foreign Secretary David Miliband last week, in which he discussed Labour's future without mentioning Mr Brown.
Meanwhile, John McDonnell, MP for Hayes and Harlington, called for would-be leadership challengers to "come out of the dark" and publicly declare themselves by the party's September conference.
Mr McDonnell said: "We desperately need to find a way out of this mess and to end the personal dog-fighting and to help restore some dignity to the standing of the Labour party and our government."
He later told the BBC that he would stand in a leadership contest, but called for the requirement for candidates to be nominated by 71 Labour MPs to be dropped - a rule that cost Mr McDonnell his challenge of Mr Brown last year when he failed to secure enough signatures.
Shadow cabinet minister Chris Grayling said the government was "in chaos at a time when Britain faces serious challenges", and called for a general election "sooner rather than later".
"The Labour Party is clearly split right down the middle with some backing Brown and others trying to knife him," he said,
Liberal Democrat Treasury spokesman Vince Cable told the Andrew Marr Show that Labour's plummeting popularity presented an opening for his party.
"It is clear that the main political opportunities are where people are looking for an alternative to the Labour government, as they do in many northern cities and in the Midlands, where the Tories are nowhere and we are the effective opposition."
Meanwhile, the Mail on Sunday has published a secret memo in which former PM Tony Blair accused Mr Brown of playing into Tory leader Mr Cameron's hands with a "lamentable" and "vacuous" performance as prime minister.
The leaked memo, which was written in the aftermath of Labour's conference last year, says the prime minister junked Tony Blair's agenda but had nothing to put in its place.
A spokesman for Mr Blair told the paper that the former prime minister was 100% behind Mr Brown, but declined to comment on the memo itself.
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