David Cameron has accused Tony Blair of presiding over a government that is "paralysed" and twice challenged him to back Gordon Brown as his successor.
Mr Brown is widely tipped as Mr Blair's successor
In the first prime minister's questions since July Mr Cameron asked Mr Blair: "Do you back the chancellor as your successor, yes or no? I mean, I do."
The PM refused to be drawn, insisting he wanted to talk about policy.
Mr Cameron said hospitals and prisons were suffering as a result of the uncertainty over Mr Blair's successor.
The heated exchanges came after questions on prisons and NHS policy.
Mr Cameron asked Mr Blair twice to say whether he backed Mr Brown as his successor, only to have the question batted back with allegations that the Tories would reverse all the reforms being made by local NHS decision-makers.
In his first attempt, the Tory leader said: "You told us in January: 'I'm absolutely happy that Gordon Brown will be my successor. He needs the confidence of knowing he will succeed me and that's fair enough'.
"Do you still think that today?"
Mr Blair said he did "not resile from anything I have said", before launching into an attack on Conservative policies on health.
Mr Cameron then asked: "Do you back the chancellor as your successor, yes or no. I mean, I do - do you?"
The prime minister replied: "I am sure you are a lot happier talking about that than talking about policy. I'm going to talk about policy on the NHS."
Finally, Mr Cameron said: "Everyone can see this government is divided and paralysed.
"We have got a prime minister who doesn't trust his chancellor; a chancellor who's been accused of blackmail; the latest home secretary wants the prime minister's job; the deputy prime minister hasn't got a job, but he's still being paid - and all the while, hospital wards are closing and prisons are in chaos.
"How many more months of this paralysis have we got to put up with?"
Mr Blair said there was "no paralysis". Under Labour there had been record investment in the health service and the government was driving forward reforms in both welfare and pensions, he said.
He then urged Mr Cameron: "If you want to be taken seriously as a leader - get serious on substance."
Mr Blair, who has led Labour to three General Election wins, has said he will step down before next September's Labour Party conference.
The overwhelming, and longstanding, favourite to succeed him as prime minister is Chancellor Gordon Brown. However there is likely to be a contest over the post.