Gordon Brown today refused to back down on controversial plans to extend the detention of terror suspects without charge to 42 days in a fighting House of Commons performance in which he labelled David Cameron a "shallow salesman".

Hours after the Prime Minister used a radio interview to admit he had initially made "mistakes" by failing to protect the poor when he abolished the 10p income tax rate, Mr Brown was asked by the Tory leader whether he would also admit defeat on the terror plans, due to come before the Commons in May.

But Mr Brown hit back, launching a robust defence of the plans, which have been criticised by Labour backbenchers, and attacking Mr Cameron for his lack of policies.

Launching his attack, Mr Cameron claimed that Sir Ken Macdonald, the Director of Public Prosecutions, disagreed with an increase in pre-charge detention from 28 to 42 days, and that the former Attorney-General and Labour backbenchers also opposed it.

"Now we know what Labour MPs themselves think about it. We’ve been sent a report from the Labour whips office. Only this Government could manage to send it across. It brings a whole new meaning to the phrase the ’usual channels’," Mr Cameron said.

"One minister says the 42-day limit has been ’plucked from thin air’. Another MP says ’I could be persuaded to stay away’. That's straight from the Prime Minister’s Book of Courage.

"Then there is this one, my own favourite, from the MP for Ealing who sums up the mood of the Labour party by saying: ’I will support it but I think it is barmy’. Why do you think you can’t persuade your own MPs?"

Launching his attack, Mr Brown accused the Opposition leader of never addressing "the substance of the issue", which was whether it was right to have the power in law that it may be necessary for the police to go beyond 28 days.

"I believe we have dealt with the civil liberties arguments in this issue. We have given the Home Secretary the requirement that she must come to the House if there is any order in any particular case.

"We have given new powers to the independent reviewer so he can adjudicate the case. We have given new powers to the judiciary so that every seven days the person has got to come before the judiciary before their detention is confirmed. I believe we are protecting the civil liberties of the country."

Sharpening his attack, the Prime Minister turned on Mr Cameron, retorting: "You are the man who wants to be both tough on crime and hug a hoodie at the same time. You are the man where political calculation meant you cycle to work but at the same time had the chauffeur-driven car coming behind.

"You are the man who is a shallow salesman and never addresses the substance of the issue. The important substance of this issue is how do we protect the people of this country against terrorism.

"That is more than trading a few quotes in the Commons. It is looking at the evidence before us and the evidence is we will need 42 days."

Earlier, in the Prime Minister's frankest comments yet over the political row over the 10p tax rate - designed to ease criticism of the Government in the run-up to tomorrow's crucial local and London mayoral polls - the Prime Minister claimed the errors the government had committed had now been corrected by policy changes, and poorer people would benefit.

The row over the decision to scrap the rate, taken in Mr Brown's last budget as Chancellor, had left Labour backbenchers threatening a mass-rebellion earlier this month which could have handed the Prime Minister his first House of Commons defeat.

Last week, Mr Brown was forced to promise urgent changes to the system to head off the row, as activists reported that the issue had dominated doorstep discussions with voters during election campaigning.

Speaking to BBC Radio 4's Today programme, Mr Brown admitted that the measures he proposed had not covered "as well as we should have" the income on OAPs and low-income workers.

"I’ll be honest about it, we made two mistakes: we didn’t cover as well as we should have that group of low-paid workers and low-income people who don’t get the working tax credit; and we weren’t able to help the 60-64 year olds who don’t get pensioners’ tax allowance," Mr Brown said.

"That is now being dealt with in the measures Alistair Darling is proposing to examine over the next few weeks and overall poorer people benefit from this Budget."

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