The Tory leader used his Webcameron site to react to the resignation
Conservative leader David Cameron has said the government is "falling apart" after the third resignation by a cabinet minister in three days.
He said Britain needed "a strong united government" and said Gordon Brown should call a general election "for the sake of the country".
James Purnell's shock resignation on Thursday night follows news that Jacqui Smith and Hazel Blears are quitting.
The Lib Dems said it was clear the government was in "total meltdown".
Mr Purnell's resignation letter - printed in Friday's newspapers - calls on Mr Brown to "stand aside" to give Labour "a fighting chance of winning".
'One final act'
Downing Street said Mr Purnell had informed Mr Brown "shortly before 10pm" - when the story broke across news outlets.
Mr Cameron, who has been calling for a general election in the wake of the MPs' expenses scandal, said a united government was needed to deal with the recession and political crisis.
"Instead we have a government falling apart in front of our eyes. Britain deserves better than this.
The people of this country have clearly lost confidence in the Labour government and it seems now that ministers are losing confidence in their droves
"With this resignation, the argument for a general election has gone from being strong and powerful to completely unanswerable."
He added: "For the sake of the country, Gordon Brown must carry out the one final act of authority left open to him, go to the Palace and call the general election we have been demanding."
Mr Purnell's resignation is the first direct challenge to Mr Brown's leadership from a cabinet minister.
Expenses allegations
In Hazel Blears' resignation letter on Wednesday she said she wanted to return to the grassroots.
But the timing of the resignation, the day before the local and European elections, raised questions - while she also did not pay the customary tributes to the prime minister, who had criticised her behaviour over expenses.
Home Secretary Jacqui Smith is understood to have told Mr Brown at Easter she wanted to stand down for her family - following expenses allegations against her. She said on Thursday he was the right man to lead the Labour Party.
For the Liberal Democrats, Danny Alexander told the BBC: "It is clear that the Labour government is in total meltdown, that Labour is finished as a government.
"The people of this country have clearly lost confidence in the Labour government and it seems now that ministers are losing confidence in their droves. We may well see more resignations."
He said it was "simply not possible to imagine" Labour continuing to govern for another year and called for a general election.
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