The odds of a terrorist bomb or plane crash in Surrey’s leafy suburbs are incredibly rare.

But when a panic button was activated at St Peter’s Hospital, Chertsey, today it was clear from staff leaping into action they are suitably prepared for a large-scale accident.

At 6am it was reported a double decker bus, carrying up to 73 people had crashed in to a bridge on the Wisley junction of the M25.

With two passengers dead and 36 others, including six seriously injured heading by ambulance to St Peter’s, a major incident was declared.

Doctors, nurses, porters, press liaison officers and other medical staff were phoned or paged as they slept and told to come in immediately.

One dedicated nurse abandoned her trip to Paris while another, who had only got back from holiday hours before, managed to get in just half an hour after being called.

Only a few people at the hospital knew before hand that this “major incident” was not actually real but a top secret training exercise.

Nevertheless, it was treated with the same seriousness as a real incident, as doctors ran out to transfer patients from ambulances and went through the usual procedures like fitting drips.

Amrat Cobb, a nurse in accident and emergency, said: “We had to pretend to put drips on whereas in a real situation it would be we would be able to do it faster rather than pretend.

“It was very beneficial because if you have gone through it before, in a real situation you can get through it a lot quicker.”

The patients were in fact trainee police officers, with various injuries, and were briefed before on the severity of their wounds and whether they were meant to be conscious or not.

Katherine Tully, a trainee PC in Guildford got up at 4am this morning to take part in the exercise and was told she had a chest injury so could not speak because she was semi-conscious.

“We were all quite intrigued by it and looking forward to helping out,” she said.

“All the staff were quite friendly. No-one was really running around inside, it was all quite controlled.

“When I was inside a police officer was asking me questions and I think if I was that police officer I’d now know how to deal with a person in my position.

“It was very really helpful.”

Most of the morning went smoothly and the hospital was “stood down” and taken off major incident alert at 9am, as planned.

The exercise was carried out in the early hours of the morning to ensure minimum disruption to real life patients.

A few non-urgent elective operations were delayed by a couple of hours while the hospital cleared its patients from the incident but no procedures were cancelled.

Some areas of improvement mentioned early on were communication between departments and police numbers in the hospital.

Organisers said these areas will be addressed in detail at a formal de-briefing within the next week.

“We’ll learn the lessons to tidy up the process,” St Peter’s chief executive Paul Bentley said.

“The purpose of this was to test how we respond.

“It’s about knowing how the system will operate before hand.”

Detective Chief Inspector Ian Chandler concluded: “We recognise we’re getting more and more of these incidents around the world and we are preparing ourselves for a major disaster.

“This is the first time we have done this and from what I can tell it has all gone pretty well. We will definitely be taking part again.”