Britain's top counterterrorism chief resigned from the force Thursday after a blunder that almost blew a huge police operation against a suspected Al Qaeda cell in the U.K.

Assistant Commissioner Bob Quick said that he "deeply regretted" having jeopardized coordinated raids that were planned for the early hours of Thursday morning but had to be brought forward and carried out in broad daylight Wednesday afternoon.

Twelve people were arrested, including 10 Pakistani nationals on student visas and one Briton, at eight addresses across Manchester and Merseyside after a long covert surveillance operation involving both police and MI5.

Quick's error was momentary but potentially disastrous. As he arrived at 10 Downing Street to brief Prime Minister Gordon Brown on the raids he was carrying under his arm a document marked "Secret," which detailed the broad lines of the operation.

The document, in a transparent plastic folder, was only visible for about a second as Quick climbed down from his vehicle. But that was more than enough time for the powerful telephoto lenses behind the press barricades across the road.

The document revealed how many terrorist suspects were to be arrested and in which cities across the North West. It revealed that armed members of the Greater Manchester Police would force entry into a number of homes and included the operation's secret code name.

As soon as the photograph was published, Scotland Yard and MI5 moved to quash its publication via a rare D-notice slapped on media organizations. The picture had already been sent to newspapers and magazines abroad, however, where the D-notice system carries no weight, and it was clear that the operation had been put at risk.

In his resignation statement this morning, Quick said: "Last evening I contacted the Metropolitan Police Authority to inform them of my intention to offer my resignation.

"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."

Click here to read more on this story from the Times of London.