Prime Minister Gordon Brown was put under further pressure to step down tonight as Work and Pensions Secretary James Purnell dramatically announced he was quitting the Cabinet as polls closed in crunch elections.

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In a resignation letter released to several newspapers, Mr Purnell called on Mr Brown to step aside for the good of the Labour Party, saying that his continued leadership makes a Conservative victory more, not less, likely.

The senior Blairite's departure follows the loss of four ministers from Mr Brown's Cabinet in the past three days, amid reports of backbenchers collecting signatures demanding his removal.

Mr Purnell's letter read: "I owe it to our party to say what I believe, no matter how hard that will be.

"I now believe that your continued leadership makes a Conservative victory more, not less, likely. That would be disastrous for our country.

"This moment calls for stronger regulation, an active state, stronger public services, an open democracy. It calls for a Government that measures itself on how it treats the poorest in society. Those are our values not David Cameron's.

"We therefore owe it to our country to give it a real choice. We need to show that we are prepared to fight to be credible Government and have the courage to offer an alternative future.

"I am therefore calling on you to stand aside to give our Party a fighting chance of winning. As such I am resigning from Government.

The Party was here long before us and we hope it will be here long after we have gone. We must do the right thing by it.

"I am not seeking the leadership nor acting with anyone else. My actions are my own considered view, and nothing more.

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"If the consensus is that you should continue, then I will support the Government loyally from the backbenches. But I do believe this question now needs to be put."

Mr Purnell's intervention is the first direct challenge from a senior Labour figure to Mr Brown's leadership and makes his position much more precarious.

The next few days will determine Mr Brown's future, as it becomes clear whether other ministers and MPs are willing to follow Mr Purnell over the parapet or whether the Prime Minister can reassert his authority with a deft reshuffle.

Mr Purnell worked for Tony Blair as a special adviser in No 10 and has advanced swiftly through the ministerial ranks since arriving at Westminster in 2001, though he has not been mentioned as one of the foremost candidates to replace Mr Brown as Prime Minister.

He is known to be close to one of the possible front-runners for the leadership, Foreign Secretary David Miliband, though he insisted in his letter that he was not acting with anyone else.

The Stalybridge and Hyde MP's shock decision to quit is a far more damaging blow to Mr Brown than the departures earlier this week of Jacqui Smith and Hazel Blears, both of whom were expected to lose their jobs in the forthcoming reshuffle.

His resignation may embolden Labour backbenchers to put their names to an email circulating at Westminster calling for Mr Brown to stand aside, particularly now that they cannot be accused of damaging the party's chances in the local and European elections.

Mr Brown's hopes of survival may hang on a stronger-than-expected performance in the polls, where voting closed at 10pm this evening.

Eve-of-vote opinion polls had put Labour as low as 16% in the European elections, behind even the UK Independence Party.

In England there were elections in 27 county councils and seven unitary authorities, as well as three mayoral votes. There were also 72 UK seats in the European Parliament up for grabs.

While a few councils will be counting ballots overnight, the majority of local authority election results are not expected until tomorrow afternoon.

Results in the European Parliament elections will not be in until Sunday night, after polls close in countries elsewhere in Europe.

Yesterday, Hazel Blears quit her Cabinet post as Communities Secretary.

Tonight she declined to speak about Mr Purnell's resignation. "I'm just watching the television," she said when asked if she felt she had precipitated the young minister's move.