The TA has 40,000 members

Mr Hoon told the Commons he had authorised the call-up "to enable the current level of operations to be sustained".

Eighteen members of 23 MI - Military Intelligence - Company based in Edinburgh will form part of the deployed intelligence team.

The company is part of 3 MI Battalion, the TA Intelligence Corps, which has its headquarters in London.

Lieutenant Colonel Paul Fox, the battalion's commanding officer, said his troops were ready to serve.

"My officers and soldiers are trained to deliver the necessary specialist skills. They are determined to make a meaningful contribution," he said.

Most of the reservists will provide intelligence support in the UK, at the Defence Intelligence Staff and the Permanent Joint Headquarters at Northwood, but up to 40 will be deployed in Afghanistan with the British land force.

Civilian jobs

The mobilisation is expected to last for six months but the MoD said the situation would be kept under constant review.

The last time there was a compulsory call-up was during the Suez crisis of 1956.

The latest call-up is seen as a sign of the scale of the workload the MoD has taken on in the war against terror.

An order to mobilise the TA has to be passed by Parliament. Only then can volunteers be removed from their jobs and sent on long-term deployments.

The government has the power to order individual members of the 40,000-strong TA to abandon their civilian jobs for up to a year if the Ministry of Defence requires their services.

Their employers must reinstate them in their positions when they return.