Commander Janet Williams joins Eliza Manningham- Buller, the head of MI5, in the front line of anti-terrorism intelligence, and is in charge of 560 officers responsible for gathering intelligence on terrorist suspects, protecting the Prime Minister and combating espionage.

Mrs Williams took up her post last week at a time when Special Branch is running 90 counter-terrorist operations a month. Mrs Williams has never worked in the branch before but is a highly experienced counter-terrorism detective. In the 1990s, she was with SO13, Scotland Yard’s anti-terrorist branch, investigating IRA and Middle Eastern terrorism.

Now in her early 40s, Mrs Williams, who is married with children, is a graduate who joined the Metropolitan Police in 1982. She was Chief Superintendent in charge of Enfield until early this year when she was chosen to go on a course for potential chief constables.

Two other women head Scotland Yard frontline units. Detective Chief Superintendent Sharon Kerr runs the Flying Squad and Commander Cressida Dick is head of Operation Trident, which combats gun crime in the black community.

However a report two years ago showed that 95 per cent of the Branch’s detectives are male. One reason may be the traditions and work of one of the units, the nearest thing Britain has to a secret police.

It was formed in 1883 as the Special Irish Branch to fight Fenian bombers who had attacked targets including the offices of The Times, and was renamed in 1888. In its time, Special Branch has spied on Lenin, protected Sir Winston Churchill and interrogated Cold War spies. Once recruited to the branch, officers often complete their careers there.

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