Defence sources said the Special Forces Support Group (SFSG), which became operational last week, would also support the police in dealing with terrorist attacks. The unit, believed to be about 1,200 strong, will carry out diversionary attacks, provide force protection, set up defensive cordons around SAS and SBS operations and give them supporting firepower.

All the troops in the new unit will continue to wear their own cap badge but will also have a new shoulder emblem that depicts a silver dagger on a green background with a red-lined black flash of lightning running through it. The emblem has raised some eyebrows, with comments that the S-shaped flash bears a passing resemblance to the insignia of Hitler’s SS.

In addition to its wartime functions, the unit, based at St Athan near Cardiff, will have a standing role to support the police in domestic terrorism incidents.

“They will be trained for a specialised counter-terrorist role and will be called in whenever the police believe a situation is so physically dangerous that they need assistance from the army,” a source said.

A small company-size unit of less than 200 paratroopers became operational last November to support the SAS and SBS in Afghanistan, but the full unit has taken nearly two years to assemble and includes a wide range of skills.

The unit is believed to have the latest high-tech special forces equipment, including computerised satellite communications that will allow it to download intelligence from national agencies such as GCHQ and MI6 as well as the Soflam laser marking system that allows it to designate targets for RAF aircraft. The unit will train alongside the SAS or the SBS and come under the direct control of the director special forces, who is based in Regent’s Park, north London.

It will become the fifth British special forces unit, alongside the SAS, the SBS, the Special Reconnaissance Regiment, which conducts undercover intelligence missions and runs agents on the ground during operations, and 18 (UKSF) Signal Regiment, which intercepts enemy radio and mobile telephone communications.

The official formation of the new unit will be announced by John Reid, the defence secretary, this week when parliament resumes after the Easter recess.

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