The test invloves a fake poison gas attack on an underground train, and 60 police cadets played pretend victims.

Streets around Bank Station were closed to the public, as hundreds of firefighters, police and ambulance crews are put through their paces.

There are no members of the public or real chemicals involved in the exercise, which is the first of its kind.

Decontamination tents

Firefighters must rescue the "victims" then take them to special decontamination tents where they will pretend to be cleaned of poisonous chemicals.

The test is expected to go on for several hours, as more people go to hospitals, pretending to feel sick.

Nick Raynsford, Minister for London, says there is nothing for people to be worried about, and there is no real threat to anyone.