Brown's terror detention plan snubbed by the Director of Public Prosecutions

Sir Ken Macdonald said he had not asked for an increase in the current limit.

He told MPs: "We are satisfied with the position as it stands at moment."

In a further blow to the Government, former Attorney General Lord Goldsmith said he had seen no evidence to support the hugely controversial move towards holding suspects for 56 or 58 days without charge.

Lord Goldsmith also revealed that if Tony Blair's government had won its bid to set a 90-day limit, he would have resigned.

The interventions left the Prime Minister looking still more isolated in his effort to secure draconian new powers for the police.

The comments by Sir Ken are the most serious blow yet.

In charge of prosecutors, he could have been expected to lead the calls for police to have as much time as possible to gather evidence before a decision has to be made on charging a suspect.

Sir Ken backed the last extension, from 14 to 28 days.

But his remarks now suggest strongly that the CPS considers the 28-day limit is adequate.

Without the backing of any key public figures in the debate - except for Scotland Yard Commissioner Sir Ian Blair - Mr Brown faces a huge battle to secure Parliamentary backing.

Mr Blair's attempt to bring in a 90- day limit ended in his first Commons defeat as Prime Minister in November 2005.

Mr Brown risk a similar fate unless he can win the support of reluctant Labour MPs.

The alternative would be to beat a humiliating retreat, after publicly insisting an extension is crucial.

Such a move would further damage the credibility of Home Secretary Jacqui Smith, who was told to secure a compromise with MPs.

Shadow Home Secretary David Davis said last night: "There can be no starker demonstration of the importance of this issue than that the last Attorney General would have resigned over a central plank of the Prime Minister's policy.

"Neither the Director of Public Prosecutions nor the last Attorney General have seen the evidence to go beyond 28 days.

"Terrorism will be defeated by good intelligence, professional policing and the rigorous application of British justice, not by unnecessary incursions into the freedoms and rights that British subjects have had for centuries."

Mr Brown's spokesman said last night: "The Government's position is well known.

"As set out in the Home Office consultation document in July, it is the Government's view that there is a case for considering an extension beyond 28 days as long as there are the appropriate judicial and parliamentary safeguards in place."

The new holes in Mr Brown's argument appeared a week after his Security Minister, former Royal Navy chief Lord West, admitted he was not "fully convinced" by the Premier's argument.

He was then forced to perform one of the fastest U-turns in political history.

An hour later, following a meeting with Mr Brown in Downing Street, Lord West issued a "clarifying" statement insisting he supported the extension plan.

MI5 chief Jonathan Evans briefed the Home Affairs Committee in private on the terror threat yesterday, but kept out of the detention debate. He is believed to have said MI5 should not make recommendations as it does not have the power to arrest suspects.