Labour could be "in a very bad place" at the next general election if Tony Blair does not quit as leader by May, Europe minister Geoff Hoon says.
Geoff Hoon says Mr Blair should "go out on a high" and not cling on
The party faced being "wiped out in a lot of places" in elections for English councils, the Scottish Parliament and the Welsh Assembly in 2007, he added.
This could create a dangerous "air gap" for Labour and allow a Tory recovery, he told the London Evening Standard.
Downing Street said Mr Blair was focusing on "getting on with the job".
The prime minister has said he will stand down within a year, with allies stressing that May is a likely date, to coincide with the polls being held on the third day of that month.
Mr Hoon told the newspaper there were still questions "in the context of the elections in May", saying: "A lot of people will be asking if it makes sense to him to carry on through those elections."
He raised the prospect of Labour activists jumping ship if the party performed poorly.
"It is a concern that if we were to lose badly in the local elections again, two years running, a lot of active Labour members would not be active by the time of the next general election," Mr Hoon said.
"The view among activists is that he [Mr Blair] should go out on a high. That should be a factor affecting his decision. He should do it while he's still popular."
The normally loyal minister's intervention is likely to reignite debate about Mr Blair's departure date, ahead of Labour's conference in Manchester in a week's time.
Rhodri Morgan, First Minister of Wales, and Labour's member of the Welsh Assembly in West Cardiff, called for "minimum distractions" before May.
"If you have the party leadership afterwards, as people have spoken about, in May, June, July, then you'll have a kind of phoney war before 3 May," he told BBC Radio 4's The World at One.
"On the other hand, unless you have it a long way before 3 May, then the problem is you get a diversion of effort into the leadership election."
Meanwhile former Blairite minister John Denham has attacked the government as over-centralised, narrowly-focused and lacking in policies.
"So much is funnelled through the narrow channel at Downing Street that we have become slower in responding to new policies. Some of our new policies haven't been well designed," he told GMTV, in an interview to be broadcast on Sunday.
He said the government lacked a "clear message" for voters and had lost touch with the public.
The row come as Labour backbencher and ex-Downing Street aide Jon Cruddas said he was considering a bid for the party leadership.
He said he had been approached by key trade union figures who wanted him to stand and was "flattered".
But he insisted that major reforms were needed, and whoever won the post should not also be given the title of Deputy Prime Minister, as John Prescott had been.
"Right now I am not crunching any numbers," he wrote in the Guardian.
"That would be presumptuous. There is no vacancy.
"I am trying to see if I can start a rolling debate about what this job is, and how the party needs to organise itself.
"One thing is for certain: the status quo is not an option."
Northern Ireland Secretary Peter Hain has already announced a challenge for the deputy leadership, when a vacancy arises, and Constitutional Affairs Minister Harriet Harman has said she is considering a bid.