An investigation was beginning today after four people were injured when a runaway train collided with another passenger service in West Sussex.

The train, which had broken down and was being shunted into Chichester station, broke free and ran into a second passenger service already at the platform.

Four people on yesterday's 12pm Cardiff to Brighton service, operated by Wessex Trains, suffered minor injuries including whiplash and bruising and were treated at the scene by paramedics.

The accident, described as low-speed, happened at 6pm.

British Transport Police treated the collision as a full-scale emergency as reports of injuries came in.

The injured people suffered minor cuts and bruises.

The Wessex Trains spokesman said last night: "We are saddened that this incident has taken place. There will be a full investigation and we will await the outcome of that."

The spokesman added: "Our train had failed in the Chichester area this afternoon. It was undergoing a manoeuvre with another train when the two trains became separated.

"At this stage we do not know why. They were attempting to couple the two trains together.

"Our train moved away and came into collision with a stationary train farther down the track which was not involved in the manoeuvre. We understand there were a small number of injuries on our train."

The spokesman said an internal investigation, which would involve the Health and Safety Executive, would begin immediately. It is also possible the HSE will conduct its own external inquiry.

It is not thought anyone on the stationary train was hurt in the crash, a Sussex Police spokesman said.

Simon Gumbrell, 34, an electronics engineer from Shoreham, was sitting at the front of the Brighton-bound train when it crashed.

At first he thought he had escaped injury, but later he went to casualty suffering from whiplash.

He said: "We pulled up outside Chichester after breaking down. A train going to Reading tried to move us from the front, but only pushed us further down the track.

"Then a train came from behind and started to shunt us into the station.

"A lot of people were very upset and frightened. A guard came out and told us to grab hold of any small children and brace ourselves for impact.

"We all looked at each other in fear as we had no idea what to expect.

"When the train slammed into the back people were thrown forward in their seats. We were thrown about again when we hit the train in the station."