David Cameron went on the offensive during Prime Minister's Questions (Reuters / Olivia Harris)

David Cameron branded Labour leader Ed Miliband "completely incompetent" in heated exchanges over the government's cut to the 50p tax rate.

His resolve boosted by falling unemployment figures, the prime minister turned the tables on Miliband during Prime Minister's Questions.

MIliband attacked chancellor George Osborne's budget, which he described as an "omni-shambles", and asked Cameron to confirm whether cutting the top rate of tax to 45 pence would be worth £40,000 to Britain's millionaires.

Cameron claimed that an amendment to the finance bill tabled by Labour and to be debated later that evening would get rid of the 45p rate and replace it with a 40p rate.

"[MIliband] hasn't had much to do over the last month. Some of us have been quite busy while he has had almost nothing to do. But even in those things that he has to do he is completely incompetent," he said.

Cameron also rounded on Miliband for failing to pass comment on UK unemployment dropping to a rate of 8.3 percent.

"I notice [he] doesn't ask about unemployment," he said to cheers from the coalition bench. "Every month when it's risen he had hopped up to leap on the bad news."

Miliband responded by claiming that "only this prime minister could think it would be a cause for celebration that more than one million young people are still out of work".

"It's no wonder people think he is out of touch," he added, before asking Cameron to confirm that the so-called "granny tax" would see 4.4 pensioners lose up to £320 a year while other tax changes would make some families more than £500 a year worse off.

Cameron resisted being put on the defensive and repeated his claims that Osborne's budget was based on increasing tax payments made by the rich while cutting the payments of 24 million working people.

He then turned his attention to the ongoing debate over Labour candidate Ken Livingstone's tax.

"[Miliband] can't even get his own Labour candidate for mayor of London to pay his taxes," he said and askede Miliband to condemn Livingstone for "paying less tax than the person who cleans his office".

"[Cameron] is very excited today," Miliband responded. "In case he is has forgotten, this is Prime Minister's Questions. I ask the questions and he is supposed to answer them."

He attempted tofocushis attacks on the government's controversial proposals to cut tax relief on charitable giving, which he claimed would cost Britain's charities up to £500m.

Cameron pointed out the Labour leader's lack of comment on Livingstone, before arguing that the cut proposed making the richest people in country pay more tax, includingtheir charitable donations.

He criticised Miliband for his failure to come out against Unison during debates over a proposed fuel strike and pointed out Labour's recent high-profile loss of its Bradford West seat.

"[Miliband] has given at least one person a job opportunity - George Galloway", he added.