Britain's top counter-terrorism officer resigned today after his security blunder threatened a major anti-terror operation. London Mayor Boris Johnson said he had accepted Assistant Commissioner Bob Quick's resignation with ``great reluctance and sadness''. Mr Quick's downfall came after he was pictured yesterday clutching sensitive documents as he arrived in Downing Street for a meeting with the Prime Minister and the Home Secretary. The documents showed top secret details of a major anti-terror operation in north west England. And the blunder meant a police operation to arrest the suspects had to be brought forward. Searches were continuing at addresses in the north west today after hundreds of officers carried out the raids on 10 properties and arrested 12 men - including 10 Pakistani nationals on student visas and one Briton.