Mr Clarke says Labour is facing 'utter destruction' at the polls
Ex-home secretary Charles Clarke has said Gordon Brown has a matter of months to improve the standing of the Labour Party or quit as prime minister.
Mr Clarke told the BBC "many, many, many people" shared his concerns.
He spoke as Mr Brown prepares to continue his autumn fight back with a speech to business leaders.
Schools secretary and key Brown ally Ed Balls dismissed the attack as "Charles being Charles". Minister Phil Hope said Mr Clarke should "shut up".
Mr Clarke has a track record of criticising Mr Brown and was one of only a handful of Labour MPs not to publicly back his bid for the premiership last year.
He has been touted as a potential leader himself, but he earlier ruled out a challenge to Mr Brown in an interview with BBC Radio 4's Today programme.
Asked if he would be prepared to stand against the PM as a "stalking horse" - to flush out more heavyweight challengers - he said: "I wouldn't do that myself - and I wouldn't counsel anybody else to do it either."
Speaking later to BBC Political Editor Nick Robinson, he said he lacked the support for a bid himself but would "urge a leadership contest," adding there were "five or six people of quality" who would make good candidates.
Mr Clarke launched his latest salvo against the PM in an article for The New Statesman, writing that Labour faced "utter destruction at the next general election" if it continued on its current course.
He stressed that there was no "Blairite plot" against Mr Brown but there was a "deep and widely shared concern" in the party that Labour was heading for "disaster" coupled with a determination "that we will not permit that to happen".
'Question of months'
Asked on Today what Mr Brown had to do, he said "establish his authority and set a very clear leadership direction".
And he said the government's performance must improve "significantly" or Mr Brown should "stand down as prime minister with honour and have a proper leadership election and address the proper issues".
Asked how long he gave Mr Brown, the former home secretary said: "I think it's a question of months really."
Labour MPs have lined up to defend Mr Brown, with one of his closest allies, Ed Balls, telling GMTV: "It's not the first time Charles has made those kind of comments. I think it's Charles being Charles.
"I don't think that's where the debate will be when we get to the next general election."
Labour deputy leader, Harriet Harman, often named as a potential challenger to Mr Brown, said: "I don't think that there is going to be a leadership challenge - nor should there be."
And health minister Ben Bradshaw dismissed the former home secretary's attack as "Westminster tittle tattle".
Asked on BBC Radio 4's The World at One why more Cabinet ministers had not publicly backed Mr Brown, he said that might have given Mr Clarke's comments "more credibility than perhaps they felt they were worth".
Former minister Nigel Griffiths also expressed his dismay at the intervention by Mr Clarke.
He told Today: "It is not as if Charles has any alternative policies.
"In 2007 he and (former Health Secretary) Alan Milburn set up a think tank called 2020 Vision.
"It didn't think but it certainly tanked. It folded having produced not one alternative policy."
He added: "Charles' problem is this is not the first time that he has gone for the old Dad's Army 'We are all doomed' without coming up with any real alternative.
"He sort of lobs a grenade into the party and then backs away and he did have his chance ... in 2007 to come up with alternative policies, and he hasn't come up with them."
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