Jack Straw has made public his support for the prime minister
Jack Straw has said he is "absolutely convinced" that Gordon Brown remains the "right man" to lead Labour.
The by-election defeat in Glasgow East was disappointing, but it would be a "big mistake" for Labour to turn in on itself now, the justice secretary said.
It is the first time he has spoken out publicly in support of the prime minister since Thursday's defeat.
Mr Straw told the BBC he had no plans to mount a leadership bid, despite reports an ally is canvassing support.
Mr Straw, who has been touted as a potential caretaker leader, had previously made it known he wanted Labour to close ranks behind Mr Brown, without coming out publicly in support of him.
Several Sunday newspapers report that former minister George Howarth, a close ally of Mr Straw, is rallying support for an autumn move against Mr Brown.
But both Mr Straw's spokesman and Mr Howarth denied there was any attempt to mount a leadership challenge.
However, the BBC has learned that some senior Labour Party figures, including some former ministers, are considering possible options for unseating Mr Brown.
'Summer of introspection'
BBC political correspondent Iain Watson said there were very active discussions in some circles about how to persuade Mr Brown to resign by the autumn.
But he added that attempting to create a vacancy was very different from trying to mount a challenge.
In his statement, issued late on Saturday, Mr Straw said: "I am absolutely convinced that Gordon Brown is the right man to be leading the Labour Party.
"I was convinced of that when I was his campaign manager last year and nothing that has happened since has changed that view.
"The result in Glasgow East was obviously disappointing but it would be a big mistake for the Labour Party to now turn in on itself and indulge in a summer of introspection."
He added: "Gordon Brown is the best leader to lead us through these tough times. He has done so before and he will do so again."
Mr Brown, who is under increased pressure after Labour lost the Glasgow East by-election to the SNP on a 22.54% swing, has dismissed calls for his resignation.
He said on Saturday: "I'm getting on with the job and I think it's important that in difficult economic circumstances we take the right decisions for the future to get fuel prices down, to get food prices down, to make sure we get the housing market moving."
Mr Brown has now gone on holiday with his family, but speculation about his future is threatening to overshadow proceedings at Warwick University, where Labour ministers, unionists and party activists are spending a final day discussing Labour's next election manifesto.
Our correspondent says the talks have already produced some concrete proposals.
Examples include extending rights for parental leave, increasing patient choice in the NHS and expanding jobs in environmentally friendly industries.
However some ministers and former ministers are concerned that their message will not be heard because of the below-par performance of the messenger, our correspondent added.
He said Mr Brown would be likely to try to seize the initiative when he returned from his break by planning a Cabinet reshuffle and policy relaunch.