Britain's top counterterrorism officer resigned on Thursday after inadvertently revealing details of confidential documents the day before.

Assistant Commissioner Bob Quick, who served as the senior police counterterrorism officer for Britain, was photographed on Wednesday carrying confidential documents into a meeting with Prime Minister Gordon Brown at 10 Downing St.

The photographs show a dossier headlined "Briefing Note: Operation Pathway" and is marked "Secret." Legible text in the pictures described the operation as a "Security Service-led investigation into suspected AQ [al-Qaeda] driven attack planning within the U.K."

Police said they were not aware of any instances in which the image of Quick holding the documents was made public before the raids, which arrested 12 men in northern England, were carried out Wednesday evening. But after the raids took place, television news reports showed images of Quick holding them.

Slip-up posed risk, officer admits

"I have today offered my resignation in the knowledge that my action could have compromised a major counterterrorism operation," Quick said in a written statement.

"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 time scale."

Metropolitan police Commissioner Paul Stephenson said Quick apologized for the lapse and "accepted that he made a serious error and that has led to his resignation this morning."

Assistant Commissioner John Yates will replace Quick as head of counterterrorism.

Yates has been involved in several prominent cases, including an investigation into whether honours such as knighthoods and seats in Britain's House of Lords were being given in exchange for Labour Party donations.

Conservative MP Chris Grayling said that Quick did the right thing by resigning.

"It is unacceptable for Britain's most senior anti-terrorist officer to have had such an extraordinary lapse in judgment. To put the security of his police officers and the operation at risk has rendered his position untenable," Grayling said.

Home Secretary Jacqui Smith, whose department is in charge of policing, said Quick felt his position was untenable, although she said the anti-terror sweep was successful.

12 arrested

British police arrested 12 men under the Terrorism Act Wednesday night in a series of raids involving hundreds of officers across northwest England, including Liverpool and Manchester.

Police said the detainees range in age from a youth in his mid-teens to a 41-year-old man.

The British government currently assesses the country's terror threat level as "severe," the second highest of five possible ratings.

It has been at that level or higher since suicide bombers with ties to Pakistan killed 52 commuters on London's bus and subway system on July 7, 2005.

With files from the Associated Press