More than 200 staff from Oxfordshire emergency crews took part in a terrorist training exercise yesterday in which nerve gas was released on a coach on the M40.

Although those taking part were not dealing with a real-life incident, managers said lessons were learned from the largest exercise of its kind to take place at Oxford's John Radcliffe Hospital.

Emergency crews worked alongside staff at the hospital in Exercise Orpheus II, which was overseen by the Government's Health Protection Agency.

The scenario was a horrific one - patients had been exposed to the deadly chemical Sarin which was used in an attack on the Tokyo underground in 1995.

Gas victims were brought to the hospital and staff were then asked to find out what was wrong with them before providing appropriate treatment.

Rosemary Dooley, deputy director of operations at the hospital, said: "We are the first trust in the country to look at how staff would respond to a nerve agent attack.

"It's the biggest exercise of this kind in a working hospital and we tried to ensure normal patients could come and go without their day being interrupted.

"This was an excellent opportunity for the trust to test the hospital and health care staff's response to a major incident in real time.

"I was nervous at first about staging this, but as time went on I realised there was lots to be gained from going ahead with it.

"We can refine our plans as part of the debriefing and learn much more than in a paper exercise.

"I think the public will be reassured by that."

Fifty casualties were played by actors, medical students and hospital staff. After arriving at the hospital they were "decontaminated" in specially-provided showers.

Once they had been though that process, they were dressed in high-visibility suits before being admitted to the hospital wards for treatment.

Ambulance teams, hospital staff and other emergency services were also dressed in chemical-proof suits as part of the dramatic scenario.

John Simpson, deputy director of emergency response at the Health Protection Agency, said: "Our field exercises give our colleagues in the NHS and the emergency services the chance to practise and hone their skills in an environment which is as close to reality as we can make it."

Mr Simpson said yesterday's "nightmare scenario" involved a terrorist travelling on a coach on the M40.

He was carrying a bottle of the nerve gas Sarin which leaked - and the driver and passengers were overcome with fumes.

There were further casualties when the coach lost control and crashed into a service station.

"Sarin was a useful substance to use in this scenario because it throws up symptoms we don't often see in clinical practice," Mr Simpson added.