Britain's most senior counter-terrorism officer has resigned after making a security blunder which caused an anti-terror operation to be brought forward, BBC reported.
Assistant Commissioner Bob Quick inadvertently revealed secret papers to photographers when arriving for a Downing Street briefing on Wednesday.
Mr Quick said he "deeply regretted" the disruption caused to colleagues and was grateful they had reacted so quickly.
Twelve men were detained in raids in Manchester, Liverpool and Lancashire.
In a statement, Mr Quick said: "I have today offered my resignation in the knowledge that my action could have compromised a major counter-terrorism operation.
"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."
Mayor of London Boris Johnson said it was "with great sadness" that he had accepted Mr Quick's resignation.
He told BBC Radio 4's Today programme the senior officer had a "very, very distinguished" career and that the memo incident had been "extremely unfortunate".
There had been no witch hunt or effort to hound him out, he said.
The mayor confirmed Assistant Commissioner John Yates would replace Mr Quick as head of counter-terrorism.
The senior officer had faced intense criticism from opposition politicians after revealing the confidential document to photographers after arriving for a Downing Street briefing.
The memo, marked "secret", carried an outline briefing on an on-going counter-terrorism operation.
It contained the names of several senior officers, locations and details about the nature of the overseas threat.
In a statement, Home Secretary Jacqui Smith said she had been informed of Mr Quick's resignation by Metropolitan Police Commissioner Sir Paul Stephenson.
"Although the operation was successful, he felt that his position was untenable. I want to offer my sincere appreciation of all the outstanding work he has done in this role," she said.
Shadow home secretary Chris Grayling said resigning was "the right thing to do".
"It is unacceptable for Britain's most senior anti-terrorist officer to have had such an extraordinary lapse in judgement," he said.
"To put the security of his police officers and the operation at risk has rendered his position untenable."
Mr Quick had been attending a Downing Street meeting in his role as lead for counter-terrorism and for the Association of Chief Police Officers (Acpo) when he stepped out of a car clutching the confidential papers.
Some hours after the Downing Street incident on Wednesday, hundreds of officers from the north-west counter-terrorism unit carried out a series of raids.
Security expert Peter Taylor told the BBC there were concerns a terror cell had been formed which was ready to attack, possibly using an improvised explosive device.
Searches are continuing at addresses in north-west England after hundreds of officers swooped on 10 properties and arrested 12 men.
Ten of those arrested are Pakistan-born nationals on student visas and one is a UK-born British national.