Britain's most senior counter-terrorism officer has resigned following a security blunder that forced police to bring forward an operation to prevent a suspected al-Qaeda plot.

Hours after Scotland Yard's assistant commissioner, Bob Quick, was photographed with a document revealing plans to arrest a group suspected of plotting attacks in the country, police arrested 12 men in unusual daylight counter-terrorism raids.

The men arrested in the raids across northwest England were mostly Pakistani nationals.

"I deeply regret the disruption caused to colleagues undertaking the operation and remain grateful for the way in which they adapted quickly and professionally to a revised timescale," Quick said in a statement on Thursday.

Quick was pressurised to resign by opposition Tory politicians who clashed with him last year when he ordered the arrest of their immigration spokesman as part of an investigation into leaks of government information.

'Lapse in judgment'

"It is unacceptable for Britain's most senior anti-terrorist officer to have had such an extraordinary lapse in judgment," said Chris Grayling, the opposition Conservative home affairs spokesman.

"To put the security of his police officers and the operation at risk has rendered his position untenable."

The document, marked "secret", described the operation as a "Security
Service-led investigation into suspected AQ (al-Qaeda) driven attack planning within the UK".

It said 11 people were targeted for arrest, 10 of them Pakistani nationals in Britain on student visas, and one British.

Quick is to be replaced by assistant commissioner John Yates, a high-profile officer who led an investigation into alleged political corruption that overshadowed the final months in office of Tony Blair, the former prime minister.

Fifty-two people were killed in July 2005 in suicide bombings on London's underground and bus network.