The day Gordon Brown promised Ed Balls the keys to No. 10

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.

The alleged promise, made when Labour looked like winning an Election comfortably, would have seen the PM standing down after serving one full term in Downing Street.

But if, as polls predict, there is a strong Conservative performance, such a pledge would immediately fire the starting gun on a Labour succession race.

A second MP said: "I have heard this before from within the Brown camp. The message was that he would fight only one Election and there would be promotion for a number of younger people."

The claim carries echoes of the "deal" made in 1994 at London's Granita restaurant between Tony Blair and Gordon Brown, when Mr Blair is said to have promised to relinquish the Labour leadership after an agreed period of time: he was then accused by Mr Brown's supporters of reneging.

Despite the so-called "Brown/Balls" deal, the Prime Minister's departure would almost certainly trigger a fierce leadership battle between Schools Secretary Mr Balls, Foreign Secretary David Miliband and Work and Pensions Secretary James Purnell.

The three have already clashed in the Cabinet as they try to outmanoeuvre each other for the right to succeed Mr Brown.

The alleged private assurances from Mr Brown could explain why Mr Balls has appeared so active in recent weeks – leading to friction with his Cabinet colleagues.

Last week, Justice Minister Jack Straw was forced to deny astonishing claims that he had threatened to punch Mr Balls for "speaking rudely" to him. Last night, a senior member of the Brown camp said the notion that Brown had set a time limit on his premiership was a piece of "ridiculous nonsense" being deliberately spread by the Prime Minister's enemies.

"There is a group of people who are trying to undermine Gordon by attacking Ed Balls," the aide said.

"The premise of a time limit is completely untrue. Gordon has been careful not to make the mistake of signalling his intentions in advance.

"This is the work of the Ultras."

The "Ultras" is the name given by Mr Brown's tight-knit team to the most loyal supporters of Tony Blair.

Rumours that Mr Brown has put a deadline on his own premiership last night also sparked comparisons with his predecessor's decision in September 2004 to say he would fight just one more Election as Labour leader.

Mr Blair was hoping to defuse mounting pressure from the then Chancellor's supporters by going public with the pledge but it left him severely weakened for his remaining time in office and open to repeated "lame duck" attacks.

One MP said: "Gordon hasn't said this in public but even if he hinted at it, it's a terrible mistake to go down the same route."

In recent weeks, to the irritation of his ambitious colleagues, Mr Balls has been at the centre of a sudden flurry of political activity, including a controversial attack on the admissions policies of faith schools – which he later admitted was based on "unverified desk research" – and a much-ridiculed photo-opportunity on a swing to promote a "play strategy". Officials in his department complain that Mr Balls is so caught up in the political in-fighting that he is "not focused on the job, just his job". The mood in the Brown camp is sure to have darkened further over a strongly-worded article in the Labour-supporting Guardian which featured a series of comments from unnamed Labour MPs.

They included: "Psychologically, Brown is brooding in a very bleak place"..."He's clinically depressed" ..."The old demons that warned he might not be up to the job are gnawing at him again"...and "There's no sense of direction whatever. There's nothing there". A comment piece by former Tory MP Matthew Parris in The Times used very similar language, describing as "unforgivable" the "empty space in Mr Brown's head where an idea ought to be".