Mr Straw, tipped as a possible replacement for the under-fire PM, has urged them to close ranks and not engage in damaging "navel gazing", sources said on Saturday.

While he found the SNP's sensational victory in the supposedly rock-solid Labour seat "very worrying", the message from voters was to focus on policy not "frantic introspection", he believes.

"He believes Gordon Brown is the right man to lead us through this," the source said. "He has led us through difficult periods in the past and can do so again."

Mr Straw spoke out amid reports that colleagues were plotting behind the scenes, with one unnamed colleague telling The Telegraph it had "moved from possible to probable that Gordon will be toppled".

Cabinet minister Ed Miliband on friday night dismissed the reports and said there was a "collective responsibility not to turn inwards but to turn outwards and understand the concerns of the country and to develop the policy agenda around the economy, around some of the social issues, around climate change - the big issues of the future that can win us the next election".

He is at the heart of this weekend's discussions of Labour's National Policy Forum in Warwick, where unions and party activists are shaping its next election manifesto.

Unions hope to use Mr Brown's weak position - and their financial leverage as providers of 90% of party funds - to force through as many of their demands as possible.

Mr Brown has consistently rejected any return to 1970s-style industrial relations such as secondary picketing.

But the Tories are set to paint him as the "helpless prisoner" of union bosses.