Minister are said to be considering emergency laws to allow police to cordon off whole sections of cities in the event of terrorist attacks.

The Cabinet Office confirmed that the government may include the legislation as part of the forthcoming Civil Contingencies Bill.

The powers may also include the authority to bar people from leaving cities if they are at risk of carrying infectious disease from a chemical or biological attack.

Specially-trained forces will be positioned at rail stations and at key roads junctions, and will have powers to deal with panic and looting.

A "semi-live" test of procedures is to be conducted in London in January.

"There are legal issues around the setting up of health cordons and keeping people in and out of them," said a Cabinet Office spokesman.

The Department of Health has already issued advice to GPs on how to detect the early signs of smallpox, to prevent a pandemic.