Britain's most senior counter-terrorism officer quit on Thursday after his security blunder forced police to bring forward a major operation to thwart a suspected al Qaeda plot.
Scotland Yard's Assistant Commissioner Bob Quick was photographed entering Prime Minister Gordon Brown's residence on Wednesday openly carrying a secret document revealing plans to crack down on a group suspected of plotting attacks in Britain.
Hours after he and the document were pictured, police arrested 12 men in unusual daylight counter-terrorism raids across northwest England. Most of those held were Pakistani nationals.
"I have today offered my resignation in the knowledge that my action could have compromised a major counter terrorism operation," Mr Quick said.
"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."
Opposition politicians, who clashed with Mr Quick last year after he ordered the arrest of their immigration spokesman, said they believed he had to go.
"It is unacceptable for Britain's most senior anti-terrorist officer to have had such an extraordinary lapse in judgement," Conservative Home Affairs spokesman Chris Grayling said.
"To put the security of his police officers and the operation at risk has rendered his position untenable."
The document, headlined "Briefing Note: Operation Pathway" and 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 Pakistan nationals in Britain on student visas, and one British.
Fifty-two people were killed in July 2005 in suicide bombings by Islamists on London's underground and bus network.
Mr Quick will be replaced by Assistant Commissioner John Yates, a high-profile officer who led a probe into alleged political corruption that overshadowed the final months of former Prime Minister Tony Blair's time in office.
Suspects questioned
Detectives were questioning the 12 suspects arrested during the raids by hundreds of officers across northwest England.
One was held at Liverpool's John Moores University, two at a home improvements store in Clitheroe, Lancashire, and one on a motorway in Manchester. Four others were held in Liverpool and four in Manchester.
The arrested men ranged from a teenager to a 41-year-old man, police said. Officers also searched eight addresses in Manchester, Liverpool and Lancashire.
