The incident at a junction beside Fisherton Tunnel at around 6.45pm involved two trains travelling in the same direction: a Great Western Railway service travelling from Southampton to Cardiff and a South Western Railway train heading from London to Honiton in Devon.

The South Western service was required to stop at a red signal, but did not and struck the side of the Great Western train, causing them both to derail while in the tunnel.

Rail Accident Investigation Branch deputy chief inspector Andrew Hall said the most likely cause of the incident was “almost certainly a result of low adhesion between the wheels and the track. We are continuing to pursue this as a line of investigation amongst others.

“Later this week we’ll be releasing the initial findings of the investigation.”

Thirteen people were taken to hospital by ambulance including the driver of one train whose injuries are serious. Thirty passengers are thought to have attended a casualty centre set up in a nearby church out of 92 on board both train services.

British Transport Police Detective Chief Inspector Paul Langley said: “This will no doubt have been an incredibly frightening experience for all those involved and our thoughts are with them and their families.

“We are working closely alongside the Rail Accident Investigation Branch and the Office of Rail & Road to establish exactly how these two trains came to collide.”

HM Deputy Chief Inspector of Railways Paul Appleton said: “My thoughts are with everyone affected by the Salisbury collision. I want to thank the emergency services and all those working in difficult conditions.”

A statement prepared by Network Rail and the two rail companies involved said everyone in the railway family is wishing the driver in hospital a full and swift recovery. “Our thoughts remain with him, and with all those affected by the incident”.

It added: “The relevant authorities are in the very early stages of their investigations, and it is likely the trains will need to remain on site for some time and the line will stay closed for at least a few days.”

(Photograph: Colin Cheesman and licensed for reuse under this Creative Commons Licence)