The case of Dhiren Barot, the most senior al-Qaeda figure to be detained in Britain, convinced police that they needed more time to hold terrorist suspects.

When Barot was arrested in August 2004, the detention limit was 14 days and detectives were dangerously close to that cut-off point when they finally found evidence to charge him. A scrap of paper with numbers and letters found in London proved to be the password to unlock computer files discovered in Pakistan that contained Barot’s plans for car and dirty bomb attacks.

In Tony Blair, the police found a political champion for their argument that the increasing complexity of anti-terrorist investigations — false identities, multiple computers and mobile phones, international travel — demanded strengthened powers of detention.

A proposal to enable suspects to be held for up to 90 days was the cornerstone of legislation tabled in 2005. In a move that did huge damage to Sir Ian Blair, the Metropolitan Police Commissioner, senior officers were sent to Parliament to lobby for the legislation.

Despite the impact of the London bombings and the backing of the two Blairs, Labour rebels joined the Opposition to vote the plan down in November 2005. It was Mr Blair’s first Commons defeat as Prime Minister.

Instead of 90 days, an amendment was passed doubling the detention period available to 28 days.

The 90-day proposal was floated again in 2006 but failed to get off the ground, especially when Lord Goldsmith, the Attorney-General, questioned the need for it.

The idea seemed dead and buried until, after the June 2007 London and Glasgow car bomb incidents, Gordon Brown announced that he would be proposing a 42-day detention power.

The measure was included in the Counter-Terrorism Bill currently before Parliament but, as Mr Brown’s political stock fell last autumn, it quickly ran into trouble.

Ministers were dubious, the police — still stinging over the 90 days fiasco — were neutral and the Director of Public Prosecutions was hostile.

Mr Brown made the measure an issue of party discipline and personal survival and it scraped through the Commons with the help of concessions, complicated amendments and the Democratic Unionists. No sooner had it reached the Lords, however, than it met a considerable obstacle in the form of Baroness Manningham-Buller, the former head of MI5.

The issue returns to the Lords, after the summer recess, facing certain defeat.

Six months ago that would have been catastrophic for Mr Brown. Today, in a radically different political climate, giving up 42 days will be embarrassing but it is no longer make or break.

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