Gordon Brown will face his first Question Time of general election year amid Westminster gossip about a plot to oust him as Prime Minister.
Speculation that backbench critics of the PM are trying to persuade sympathetic Cabinet ministers to resign in a bid to force him out was dismissed as "nonsense" by Downing Street insiders.
But the rumours gained such a head of steam in the tea rooms of Westminster and in the blogosphere that Olympics minister Tessa Jowell was forced to put out a statement denying she was planning to quit.
Some commentators had suggested that Blairite Ms Jowell could be the subject of rumours of an impending resignation, because of her recent attack on the "hideous" class-war strategy of highlighting David Cameron's Eton background.
But Ms Jowell said: "This story is complete and utter rubbish and I have no intention of resigning."
Lord Mandelson warned Labour against an election strategy targeted on the party's traditional core support among the working classes, insisting: "We are not a sectarian party. We are not a heartlands-only party... We are not going to win an election on that basis."
The Business Secretary - widely seen as deputy prime minister in all but name - said that Labour had to "reach out to the whole of the New Labour coalition that brought us support in the last three elections and without which we will not win the next".
MPs returned from their Christmas break on Tuesday following a festive recess marked by a string of calls from anti-Brownite backbenchers Charles Clarke, Barry Sheerman and Greg Pope for Mr Brown to be removed as PM early in the New Year.
Mr Brown's Labour critics feel that an alternative leader would have to take over within weeks to have any chance of making a dent in the Tories' double-digit opinion poll lead by the final possible election date of June 3.
But repeated polls suggest that there is no clear candidate among Labour's senior figures who could be guaranteed to turn the party's position around if he or she took over as PM.
