An SAS unit is now for the first time permanently based in London on 24-hour standby for counter-terrorist operations, The Times has learnt.

The basing of a unit from the elite special forces regiment “in the metropolitan area” is intended to provide the police with a combat-proven ability to deal with armed terrorists in the capital.

The small unit also includes surveillance specialists and bomb-disposal experts.

Although the Metropolitan Police has its own substantial firearms capability, the fatal shooting of Jean Charles de Menezes, the Brazilian electrician who was mistakenly identified as a terrorist bomber on the run, has underlined the need to have military expertise on tap.

Defence sources emphasised that a minister would have to make the decision to use the unit based in London. The Home Office would make a request to the Ministry of Defence, which was the usual procedure when the military was required to help in a police operation, the sources said.

The SAS counter-terrorist unit in London is part of an expanded headquarters in the capital for the Hereford-based regiment, which has been given extra resources and manpower to deal with the rising threat from international terrorism.

More funding has been provided to ensure that the four squadrons of 22 SAS, the regular army special forces regiment, are fully manned. Its smaller counterpart in the Royal Marines, the Special Boat Service (SBS), has also been given more money and is now in the process of gaining a second squadron.

Since the 1970s, the SAS has maintained a round-the-clock, counter-terrorist squadron based at Hereford, its main headquarters.

It has been drafted in from Hereford on several occasions to assist in terrorist incidents, notably the Iranian Embassy siege in 1980 and the Balcombe Street siege in 1975, when an IRA active service unit held a couple hostage in their flat.

However, with the expansion of the special forces in the past two years, particularly after the July 7 suicide bombings in London in 2005, it was considered prudent to have an SAS counter-terrorist unit located full-time in the capital.

The Ministry of Defence requested that the location of the unit be kept secret.

The Director Special Forces or a senior member of his staff attends Cobra, the Cabinet Office civil emergencies committee, whenever there is a suspected imminent terrorist attack or when an incident has taken place.

The presence of a standby SAS team in London has coincided with moves by MI5, which has responsibility for counter-terrorist intelligence, to expand its own workforce to meet the increasing terrorist threat. MI5 has set up a special headquarters on the outskirts of London for teams of surveillance experts to give them easier access to the North and the Midlands.

Meanwhile, according to Whitehall sources, the overarching responsibility for co-ordinating the Government’s counter-terrorist strategy may be given to the Cabinet Office, after a security review carried out by John Reid, the Home Secretary.

Sir Richard Mottram, the security and intelligence coordinator who is the Government’s principal counter- terrorist adviser in the Civil Service, is based at the Cabinet Office.

'Who dares wins'

July 2005 SAS reportedly helped police to track down bombing suspects, including taking part in a raid in West London in which they blew off a door and threw stun grenades

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