Jane Kennedy said she was not re-appointed as an environment minister because she refused to give a pledge of loyalty to the Prime Minister.
She said she told him in a "frank and honest" phone call this morning "I could not offer him the support he was asking for".
And she hit out at "smears" of colleagues by people associated with No 10. She added that if Mr Brown stayed on to the bitter end it would spell "the bitter end of the Labour Party".
Outside her constituency office in Liverpool, Ms Kennedy said: "He did not re-appoint me. My view was I was sacked. His view is that I resigned. In the end it was my choice to go."
Mr Brown's spokesman flatly denied the premier had asked her, or any other minister, for a loyalty pledge as another day of Whitehall drama unfolded.
Tonight Mr Brown was due to address a meeting of the Parliamentary Labour Party at the Commons, with Labour MPs reeling from disastrous European Parliament election results.
It was their worst electoral showing for nearly 100 years after finishing third in vote share behind the Tories and Ukip.
MPs were also dismayed that the far-right BNP gained two MEPs.
With all the results from 11 regions across the UK in, Labour had managed just 15.8 per cent of the popular vote to Ukip's 16.5 per cent.
The Tories topped the poll with 27.4 per cent of the popular vote, the Liberal Democrats finished fourth with 13.8 per cent.
Despite victories in the North West and Yorkshire, the BNP had a smaller share of the vote than the Greens, with 6.2 per cent to their 8.6 per cent.
Today's results are Labour's worst in a nationwide poll since the 1910 general election, when their leader was George Nicoll Barnes and their vote share was just 7 per cent.
Mr Brown completed an unremarkable reshuffle of ministers of state before attending tonight's meeting of Labour MPs.
Labour deputy leader Harriet Harman insisted: "I do actually think that Gordon Brown and the Government have got the best strategy for helping the economy through these difficult times, and I know people are blazing angry with us at the moment.
"They told us to our face but they want us to do better, they want us to sort out the economy and they want us to sort out what they regard as the aberration of expenses."
But former lord chancellor Lord Falconer - the most senior figure so far to break cover - repeated his call for a new leader to re-unify the party.
"I think unity will only come with a leader that the mainstream votes for," he said.
Birkenhead MP Frank Field repeated his calls for Mr Brown to go, saying: Labour cannot win with the present Prime Minister.
"I was one of the seven who would not support his coronation after Tony Blair was shoehorned out of Number 10. But even I didn't think a Brown administration would be as inept as this one."
There will be particular dismay that Labour's vote has fallen so far that it opened the door for the BNP to take seats in Yorkshire and the Humber and in the North West, where the party's leader Nick Griffin was elected.
Health Secretary Andy Burnham said it was "deeply uncomfortable" to see the BNP polling in such large numbers.
He said they had been the beneficiaries of an "anti-politics mood" which hit all the main parties in the wake of the MPs' expenses scandal.
"It is a sad moment in British politics," he said.
Speaking as he arrived at an international trade conference in London, Business Secretary Lord Mandelson said: "I think what's interesting about these results is Labour voters have not switched en masse to the Conservatives, Liberal Democrats or other parties.
"In the main, what they seem to be doing is registering a protest by withholding their vote and staying at home.
"I can understand why they're doing this.
"They are furious about the MPs' expenses and allowances at Westminster and, frankly, they're furious, too, about what they see as disunity amongst Labour MPs at Westminster.
"It is simply not possible for people to say 'I resign today, but you should vote Labour tomorrow or the day after'."
The Tories topped the popular vote in Wales for the first time, and leader David Cameron headed to the principality to congratulate party workers.
Mr Cameron, speaking outside his home in west London, said: "I am very pleased with these results. Together with the local elections, I think they show an enormous gap opening up between Labour and Conservative - almost getting twice as many votes as Labour last night.
"Now what we need is obvious, the next election should be a general election, and just as Labour has lost the trust of the British people, I want the Conservative Party to work hard to win that trust.
"Just as Labour has failed, we have to work hard to show how we can succeed."
He added: "One of the reasons we want a general election is that the British public are angry that they are being locked out of passing judgment on this whole expenses scandal. The longer we put off an election, the greater that anger will be."
Ukip leader Nigel Farage brushed off suggestions that his own party's second place was a result of Labour's unpopularity rather than voter enthusiasm for Ukip.
Mr Farage told BBC Radio 4's Today programme: "According to all the experts, this is the second fluke in a row that we have produced."
Senior Liberal Democrat Simon Hughes said his party had held its ground while there had been a "significant loss" for Labour.
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