Britain's top counterterrorism officer resigned from London's Metropolitan Police on Thursday - one day after a security blunder forced police to move up a major operation in northern England. Assistant Commissioner Bob Quick, who was the senior police counterterrorism officer in Britain was photographed Wednesday clutching confidential documents that could clearly be seen as he arrived for a meeting with Prime Minister Gordon Brown at 10 Downing St. The documents showed details of the planned anti-terror operation in northern England. In anti-terror raids later on Wednesday, police arrested a dozen men. Police said they were not aware of any instances in which the image of Quick holding the documents was made public before the raids were carried out Wednesday evening. After the raids took place, television news reports showed images of Quick holding them. Thursday morning newspapers also carried the photos, with at least one major daily showing a close-up image of the documents in which details of the operation were legible. "I have today offered my resignation in the knowledge that my action could have compromised a major counterterrorism operation," Quick said in a statement released by the department. "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." Commissioner Paul Stephenson, who heads the Metropolitan Police, said Quick "accepted that he made a serious error and that has led to his resignation this morning." Assistant Commissioner John Yates is replacing Quick as head of counterterrorism, Police Commissioner Paul Stephenson said. Yates has been involved in several prominent cases, including an investigation into whether honors such as knighthoods and seats in Britain's House of Lords were being given in exchange for Labour Party donations. At the end of the investigation in 2007, which included questioning Tony Blair who was prime minister at the time, prosecutors did not charge anyone. Opposition lawmaker Chris Grayling said 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. Quick previously was criticized for his role in the decision to arrest opposition lawmaker Damian Green last year during an inquiry into alleged leaks of sensitive information from the Home Office. Conservative lawmakers were upset about the arrest of Green, who was never charged, and the search of his office in Parliament. Home Secretary Jacqui Smith, whose department is in charge of policing, said Quick felt his position was untenable after his mistake on Wednesday, although she said the anti-terror sweep was successful. Hundreds of officers across northwest England were involved in the anti-terror raids Wednesday evening. Greater Manchester Police said the suspects were detained under anti-terrorism laws at addresses in the cities of Manchester and Liverpool and the surrounding area, about 200 miles (300 kilometers) northwest of London. Police did not provide details about why they conducted the raids except to say they "acted on intelligence received." They said the suspects ranged 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 four suicide bombers killed 52 commuters on London's bus and subway system on July 7, 2005.