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.