Footage from the scene showed at least three overturned carriages and paramedics on site. The cause of the incident remains unknown.

Deputy chief paramedic officer Michael Bradfield said at a news briefing that there are a “range of injuries” among the 20 people that were taken to hospital. Of those injured two “were categorised as severe and were conveyed to a major trauma centre”.

Ian Drummond-Smith from the British Transport Police said a “major incident” had been declared but that all passengers on the train had been “safely evacuated”.

The accident involved a train from Southern Rail, which connects areas south of London to the capital.

Network Rail’s route director for Sussex, Lucy McAuliffe, said they are “working closely with the Rail Accident Investigation Branch, who are on site to establish exactly what has happened”.

Train accidents, particularly fatal ones, are relatively rare in Britain.

A train driver was killed and dozens of people injured when two trains collided north of London in June.

Four people suffered minor injuries in a train derailment due to a reported landslide in remote northwestern England last November.

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