A chemical strike on a busy Tube train will be simulated in central London on Sunday.
Police have stressed that no real chemicals will be used
Police cadets playing the part of victims will be decontaminated on the street as part of a major counter-terrorism exercise on the Waterloo and City line.
The government has been giving more details of the exercise which has been planned for some time but was delayed because of the war in Iraq.
There have been exercises testing the theoretical response of the NHS and different command centres, but this will be the first live operation in London.
It is also the first time the decontamination units have been seen in public.
The exercise will not involve members of the public or real chemicals, but is designed to test the response of the police, ambulance and fire services to a terrorist attack. The Army will not be involved.
The scenario acted out will be that of a chemical released on a busy train from Waterloo to Bank Tube station, in the heart of the City of London.
The driver will report the attack and the train will stop in the tunnel just outside Bank.
Firefighters and paramedics will then recover the 'casualties', bring them to street level and decontaminate them in special shower tents.
The streets around Bank station will be cordoned off on Sunday and staff at University College Hospital will also be involved.
Passengers will arrive at the hospital later reporting feeling unwell and will be treated by medical staff in protective clothing.
The whole exercise is expected to last several hours.
Minister for London Nick Raynsford told BBC London: "We are doing everything we possibbly can to ensure that all the services in London are as well prepared as they can be to cope with any eventuality
"That's how we are trying to ensure the people of London are as protected as possible."
Other Tube lines which run through Bank station will operate as normal on
Sunday but will not stop at Bank.