An explosion in an ambulance, which rocked a hospital and forced the evacuation of 200 patients, staff and visitors, has been blamed on an isolated mechanical failure.

One firefighter was injured when gas cylinders aboard the emergency vehicle ruptured and exploded at St Helier Hospital last September.

Nearby windows were blown out and the homes of residents more than a mile away in Sutton shuddered in the blast which caused flames and thick black billowing smoke.

London Ambulance Service conducted an investigation into the incident and earlier this year submitted their report to the Health and Safety Executive, which has not pursued the matter.

However, politicians said LAS, which has yet to release its report publicly, still had questions to answer.

Conservative spokesman for health and safety, councillor Stuart Gordon Bullock said he wanted to know whether the incident was properly investigated.

He said: “It was in the national news and the local newspaper. Local people heard the bang and experienced the traffic chaos, it was horrific, and there were loads and loads of people affected.

“It’s a matter of public interest and transparency; we need to know the ambulances are being maintained properly.”

MP for Carshalton and Wallington Tom Brake said: “I want a clear statement from LAS about the action plan they are implementing to ensure this never happens again.”

At the time three members of staff suffered minor injuries, but no patients were injured.

The explosion left the ambulance as a shell and another ambulance parked opposite was also damaged.

Paul Woodrow, assistant director of operations at LAS, said: “Following an investigation, we have not identified any similar faults with these types of vehicles in our fleet, which are currently being replaced with brand new Mercedes ambulances as part of our fleet replacement programme.”

Epsom and St Helier University Hospitals NHS Trust also carried out its own internal examination of its handling of the incident.

Director of communications Antony Tiernan said it found that staff responded quickly and positively, but made recommendations to review the types and volume of hazardous gasses and substances stored on site and the role of security in the event of an internal major incident.