Gordon Brown's collapsing authority has been further undermined by claims he had secretly assured his preferred successor Ed Balls that he would fight only one General Election.

The allegations, made by senior MPs to The Mail on Sunday, were met with furious denials by the Prime Minister's supporters, who blamed "Blairite Ultras" for trying to destroy Mr Brown.

The row broke amid extreme pressure on Mr Brown's leadership – with backbench MPs warning that he could face a challenge within weeks if Labour's performance in next month's local elections is as disastrous as they fear, and Cabinet Minister Tessa Jowell breaking ranks to urge him to take the party's opposition to his tax policy "very seriously".

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His problems were deepened by what appeared to be a concerted campaign to destabilise him through the media.

The normally loyal Guardian newspaper suggested yesterday that he was "clinically depressed" and quoted an MP as saying "there's nothing there".

A columnist in The Times echoed the charge by describing "the empty space in Mr Brown's head where an idea ought to be". One Labour MP claimed last night that Mr Brown had made a pledge last year, during the handover from Tony Blair, to fight only one Election.

Mr Brown hoped the pledge would deal with a feared leadership challenge from David Miliband while assuring Mr Brown's long-term aide, Schools Secretary Ed Balls, that the prize was within his grasp.

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