One of the largest ever simulated terrorist attacks involving civilians in the UK is being staged in the West Midlands.
'Casualties' will undergo a decontamination process
Emergency planners want to test how well more than 2,000 fire, ambulance and police officers will respond to Sunday's staged poison gas attack.
Around 400 volunteers from the Army and Red Cross will be "contaminated" at a major public venue.
But the Fire Brigades Union said its members are taking part under duress.
Firefighters are still waiting for their pay award to be honoured.
But letters have gone out to more than 300 of them warning that if they do not take part in the exercise, they will be sent home without pay, the FBU said.
Casualties will undergo a decontamination procedure, which involves having their clothes cut off and entering a special disinfecting shower, with the more
seriously injured taken to hospital.
The location of the mock attack is being kept secret because it is feared that response times will be affected, if people know where to go beforehand.
Exercise Horizon, involving forces in the West Midlands, Herefordshire, Staffordshire, Shropshire, Warwickshire and Worcestershire, will last from 0800 until 1600 BST.
It follows a simulated chemical attack on the London Underground in September last year when hundreds of people were evacuated from Bank station in the City.
Since then there have been similar exercises at several places around the country, including Newcastle-upon-Tyne and Abingdon, Oxfordshire.
The regional resilience team has been planning this weekend's operation since January.
Its deputy director, Helen Braithwaite, said it will probably be the largest civilian chemical, biological, radiological and nuclear exercise of its kind in the UK.
She added: "The exercise is not in response to any specific threat.
"It's part of an ongoing programme throughout the West Midlands - and the UK as a whole - to ensure co-ordination of resources and communication between the different organisations involved.
"Everyone is aware that the threat from international terrorism remains real and serious. But the public should remain alert and not alarmed."