More than 100 staff were evacuated from Oxford's Royal Mail sorting office in Cowley yesterday following an explosion.

It is believed a gas canister may have caused the explosion, which happened in an engineering workshop at the depot in Alec Issigonis Way.

However, investigators say it is too soon to confirm the cause of the incident.

Firefighters and police officers were called to the scene at about 11.30am and staff were later evacuated as the investigation continued.

By 3pm police were satisfied the cause of the explosion was not sinister and allowed the 110 members of staff to return to the building.

Nobody was injured in the incident, although witnesses were treated for shock.

Officers have now handed the investigation over to Oxford City Council's environmental health team who will look into what caused the explosion.

Last night, public health team manager Ian Wright said: "We have now begun carrying out our health and safety investigation at the depot. It is too early to say what the cause of the incident was."

Royal Mail spokesman Richard Hall said the explosion was not related to an item of mail and damage was limited to a wall in the affected room.

He said: "The incident was some distance away from the mail sorting operation and no mail was damaged. The evacuation was just a precautionary measure."

He said about two hours of work had been lost - which could cause delays to mail deliveries today.

A Royal Mail worker, who did not want to be named, said: "We heard an almighty bang but had no idea what it was. It happened in an area which few people have access to, so we didn't think it could be a bomb. We were not evacuated from the building until some time afterwards and did not return to work until later in the afternoon when things seemed to return to normal."