Britain's top counter-terrorism officer has resigned after his security blunder caused a major anti-terror operation against a suspected Al-Qaida cell to be brought forward, reports say.

London Mayor and chairman of the Metropolitan police authority, Boris Johnson, said he had accepted Assistant Commissioner Bob Quick's resignation with great reluctance and sadness after the latter admitted that he could have compromised a major counter-terrorism operation.

Assistant Commissioner John Yates will replace him, becoming the third anti-terrorism chief in three years. Peter Clarke and Andy Hayman, retired in recent years and Quick's Number Two, Deputy Assistant Commissioner John McDowall, has been struck down by a serious and debilitating illness.

Quick's exit came after he was photographed Wednesday as he got out of a car at Prime Minister Gordon Brown's official residence in London clutching sensitive documents on which details of the undercover operation--codenamed Pathway--in north-west England could be seen, as he arrived at Downing Street for a meeting with the Prime Minister and the Home Secretary.

Johnson said that he did not pressure Quick to quit, and said that the counter-terrorism head decided to leave of his own volition. In the end, Bob Quick decided it was the best thing to do. It's matter of sadness and he had a very very distinguished career in counter-terrorism, the mayor said, while stressing there was no effort to get him out.

Home Secretary Jacqui Smith said Quick felt his position was untenable following the publication of the photographs and thanked him for his work.

She said: Sir Paul Stephenson has informed me that Assistant Commissioner Bob Quick has offered his resignation following the publication of certain photographs yesterday. Although the operation was successful, he felt that his position was untenable.

The realization by Quick that he had held the briefing document--that set out the strategy for smashing an alleged cell based in the North-West that was thought to have been plotting an attack in Britain--in open view--had prompted alarm among the security services and led to the operation being brought forward and the arrest of 12 men--including 10 Pakistani nationals on student visas and one Briton--in Manchester, Liverpool and Clitheroe. They ranged in age from a teenager to a 41-year-old man.

Reports said the alleged plotters discussed targeting nightclubs and shopping centers, thought to have included Manchester's Trafford Centre and Arndale Centre.

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