The prime minister has rebuffed David Cameron's call for a general election, saying Conservative policies consist of "talk, talk and talk".
Gordon Brown faced questions from the Conservative leader over his ability to command the cabinet.
But despite the loss of two cabinet ministers in as many days, Brown insisted he would continue to tackle the key issues facing Britain.
Cameron told MPs that Brown should "accept that his ability to command his cabinet has simply disappeared".
"If these people have done such good work, why are they walking away from their jobs?" he asked.
And he said the resignation of Hazel Blears as communities secretary was "a direct challenge to his authority".
Seeking to diffuse tensions on the Labour benches, the prime minister paid tribute to the "great work" of his departing ministers.
Brown said "the past few weeks have been difficult for every member on every side of this House", noting there had been pressures on family members as well as MPs themselves.
But Cameron said Number 10 had been "directly briefing against" ministers, adding that the prime minister "cannot give a lead".
"The prime minister has got to understand that the issue here is about his leadership," the Tory leader added.
He also sought confirmation that Alistair Darling would remain as chancellor after the reshuffle.
Brown avoided giving that reassurance, and accused Cameron of lacking policies to deal with the economy.
Cameron is "incapable of dealing with the big issues facing this country", he said, adding that their policy is "to do nothing".
"That is not the basis on which to ask for an election," Brown insisted. "This is the party which is talk, talk and talk."
But the Conservative leader insisted there should be an election, saying the government "is now collapsing before our eyes".
Brown rejected the criticism, saying there is "no substance" to Cameron's claims.
Nick Clegg told the Commons that the prime minister was "thrashing around fighting for his own political survival".
And the Liberal Democrat leader warned of a danger to democracy when people feel "no one is in charge".
"Labour is finished. Isn't it obvious that the only choice now is between the Conservatives and the Liberal Democrats?" he said.
The prime minister said the Lib Dems "have said that at every election I have ever fought".
"We are the party with the ideas about how to take this country out of recession," he added. "None of these parties have got anything to offer us."
Dods Parliamentary Communications Ltd