Gordon Brown is preparing for a humiliating climbdown over his proposal to hold terrorist suspects for 42 days after being told that it will be defeated in the House of Lords.

Ministers admit privately that there is not “a cat in Hell’s chance” of the legislation, which returns to the Lords this week, being passed into law.

The Government has decided against using the Parliament Act to force the measure through after peers reject it, The Times has learnt. That decision will effectively confine the controversial proposal — which the Prime Minister fought tooth and nail to get through a Commons vote in June — to the legislative dustbin.

The Terrorism Act 2006 increased the pre-charge detention limit from 14 to 28 days. The imminent abandonment of the proposal to extend this further to 42 days comes after mounting criticism from senior figures in the fight against terrorism.

Writing in The Times today, the former police chief who was in charge of anti-terrorism operations across Britain described the proposed mechanism for triggering the emergency detention power as “not fit for purpose”.

Andy Hayman, former Assistant Commissioner for Special Operations at Scotland Yard, gave the clearest signal yet that police chiefs are unhappy with the proposals before Parliament.

Mr Hayman said that concessions made to secure the Counter-Terrorism Bill’s passage through the Commons had created a scheme that was “bureaucratic, convoluted and unworkable”. He added: “The draftsman’s pen has introduced so many hoops to be jumped through that a police case for detaining a terror suspect will become part of the political game.

“It would have been my job to make these proposals work, but just trying to understand them gives me a headache.”

Mr Hayman’s intervention in the debate is significant. He was the first police officer informed by the Prime Minister in July last year of his plan to seek extra detention powers.

He says he was astonished that Mr Brown was revising the proposal after Tony Blair’s bruising defeat in 2005 when he tried to give the police the power to hold suspects for longer.

Mr Hayman adds: “I remain curious as to what prompted this rethink.”

He focused his concerns on the complexity of the system being proposed, which would require the involvement of a judge, the Director of Public Prosecutions and an independent lawyer before extended detention could be authorised. That would be followed by a parliamentary debate on whether there was a sufficiently grave emergency to justify the use of the 42-day power.

But he said that his opposition to the proposals did not change his view that the police would soon require extended detention powers.

Mr Hayman added: “As someone who has been deeply involved in every major counter-terrorism investigation since 2005, I am convinced that we will soon need the power to hold suspects for more than the current limit of 28 days — and that we need to legislate for that power now rather than in the middle of an emergency.”

The former officer joins other respected voices from the counter-terrorism world in openly criticising the Government’s 42-day plan.

Baroness Manningham-Buller, the former head of MI5, used her maiden speech in the Lords in July to attack the measure.

She said: “In deciding what I believe on these matters, I have weighed up the balance between the right to life, the most important civil liberty, the fact that there is no such thing as complete security and the importance of our hard-won civil liberties. And therefore on a matter of principle, I cannot support the 42-days pre-charge detention in this Bill.”

She went on to express similar concerns to those that have been voiced by Mr Hayman: “I don’t see on a practical basis, as well as a principled one, that these proposals are in any way workable.”

Sir Ken Macdonald, QC, the Director of Public Prosecutions, has stated his view that there is no need to extend detention powers beyond the present limit. “In our experience, the 28-day limit works well,” Sir Ken told The Times in April this year.

Aides to Boris Johnson, the Mayor of London, denied reports that he would try to block the appointment of a supporter of 42 days as the next Metropolitan Police Commissioner.

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