Gordon Brown's bid to become Prime Minister has stepped up a gear after Jack Straw announced he would run the Chancellor's campaign to succeed Tony Blair.
The Commons leader declared he had "no doubt" Mr Brown had the best combination of experience and vision to succeed Tony Blair.
His endorsement came amid renewed speculation of a potential challenge from within the cabinet - with the focus centred on Environment Secretary David Miliband.
And Peter Mandelson - the arch-Blairite European Commissioner - said the party needed a new leader to emerge "from nowhere" like the Tories' David Cameron.
Mr Straw, who jointly ran Mr Blair's successful Labour leadership campaign in 1994, revealed his decision in a letter emailed to MPs.
The former foreign and home secretary, also announced he would not now make a widely-expected bid to become the party's next deputy leader.
He said last week's Budget had shown Mr Brown was "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".
Support had already arrived from all parts of the party, he told the BBC, but refused to be drawn on suggestions he could be rewarded by following Mr Brown into 11 Downing Street.
He said he would begin assembling a team and conducting "preparatory work" in coming weeks and appeared to confirm that Mr Blair would announce his resignation in May.
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The PM has said he will go by September, and is widely expected to announce he is going soon after the May 3 elections to the Scottish Parliament, Welsh Assembly and local councils.
