Jack Straw is to manage Gordon Brown's leadership campaign, and will not stand for the deputy leadership of the Labour Party.
The Commons leader wrote to fellow MPs on Saturday to tell them he had accepted the invitation from Brown.
"As the Budget showed, Gordon Brown is supremely well-qualified to build on the huge achievements of Tony Blair's premiership, to keep the Labour Party united, to give us the strategic vision to take on and defeat the Tories at the next election and, above all, show the strength of leadership our nation demands," he wrote.
"I want the campaign to be one which involves the whole of the Labour Party, which binds the party and which ensures the election of the best person to lead the party and the country after the prime minister steps down.
"There is absolutely no doubt in my mind that Gordon is the right person to do this."
Straw, a former home secretary who ran Tony Blair's successful leadership campaign in 1994, had been thought to be considering joining the six declared candidates competing for the deputy leadership.
It is unclear if John McDonnell and Michael Meacher - so far the only declared leadership challengers to Brown - will gain the 44 signatures needed to get onto the ballot paper.
Straw, appearing on BBC 1's Sunday AM, said there would be a "vigorous campaign" whether Brown was challenged or not.
He appeared to confirm speculation that Blair will stand down after May's local and devolved elections, saying: "The national executive has said there will be a seven-week period, at least, following the local elections.
"That is a good opportunity for Gordon to get round the country, for Gordon to talk to people - not only party members but also to people who are supporters and people who are opposers."
Straw refused to be drawn on speculation that he could be offered the job of chancellor in a Brown government.
26 March 2007