The SAS was called in to arrest terrorist suspects being harboured in Hendon and the Mill Hill army barracks last Saturday.

The squad was joined by police and emergency services in Barnet, acting on information provided by the Home Office Emergency Planning Department.

The suspects were arrested on suspicion of waging the largest chemical biological (CB) warfare attack this country has seen since the Second World War ,, as part of Operation Trump Card, a CB warfare training exercise.

The exercise was designed to test the reactions of the capital's emergency services and communication lines between them and local authorities.

The head of the Government's information and communication service Mike Granatt said: 'The threat of such an attack is low, but however low we have to be prepared.'

The exercise kicked off at the Harlequins rugby ground in Twickenham on Saturday.

Fictional terrorists attacked an 'unsuspecting crowd' of volunteer extras at a staged festival.

Police followed the 'suspects' to an address in Hendon and finally made the arrests at the Inglis army barracks in Mill Hill.

'It was all pre-staged,' said a Scotland Yard spokesman.

'There was minimum disruption to residents. It went very smoothly and it will teach us lots of things,' he added.