PRIME Minister Gordon Brown was last night openly challenged to stand down as another minister quit the Cabinet.

The dramatic move by Work and Pensions Secretary James Purnell increases the chance of a leadership battle after he defiantly called on Mr Brown to go for the good of the Labour Party.

In his resignation letter, the senior Blairite said his continued leadership makes a Conservative victory more, not less likely.

Meanwhile, Darlington MP Alan Milburn was dramatically named as a key Labour plotter trying to topple Gordon Brown yesterday, as the Prime Minister prepared for an election drubbing that could decide his fate.

Nick Brown, Labour’s chief whip, took the extraordinary step of publicly identifying the MP and four other backbenchers “active” in the scheming to force the Prime Minister out within weeks.

The tactic – clearly designed to “smoke out” and isolate leading rebels – brought out into the open the decadelong feud between Mr Milburn and Mr Brown, previously fought behind the scenes.

And it provided concrete proof of the growing panic in Downing Street, as the Prime Minister fought for his political survival against his demoralised party.

Nick Brown told The Northern Echo: “I have been told that Alan Milburn is in the anti-Brown camp and is active in it.

“It’s pretty clear what’s going on at the moment, with Labour MPs who do not support the Government or the Prime Minister and are involved in the scheming that’s going on.”

The Chief Whip said that he could not be certain whether his information about Mr Milburn was “true or false”. However, he added: “He has got past form on this.”

Last night, Mr Milburn, who is in the Middle East with Stephen Byers , could not be contacted to comment on the allegation.

Another of the five named plotters, Paul Farrelly, hit back at what he called “black propaganda”, saying: “What Nick Brown has been saying is completely untrue.

“It’s an unacceptable style of government that, if it’s not changed, will lead my precious Labour Party into oblivion.

For what he has done he should really consider his position.”

The rumours of a formal plot have centred on an email, which Labour MPs are likely to be sent today and asked to add their signatures to.

It praises Mr Brown, but then goes on to say: “You can best serve the interests of the Labour Party by stepping down as party leader and Prime Minister, so allowing the party to choose a new leader to take us into the next General Election.”

MPs have apparently been assured that their names will not be published unless a target of 50 backers is reached.

The plotters say they expect to get 75.

Although that will not be enough to force a leadership contest, the organisers hope to stiffen the spines of Cabinet ministers, to force them to strike.

However, Nick Brown said he was not accusing Mr Milburn or Mr Byers of being behind the email, adding: “That has been been drawn up by a different group of people.”

Meanwhile, the Prime Minister’s allies were braced for Labour MPs to launch a fresh wave of attacks on his leadership after the polls closed late last night.