The ambulance service said there were around 150 passengers on board the train which departed from London Victoria station and veered off the tracks shortly before 4:00 pm local time.
Two people sustained "serious injuries" and nine more were hurt as a busy commuter train from London derailed on Thursday near the southeast English town of Lewes, emergency services said.
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"At this stage, we can confirm that two patients have sustained serious injuries and a further nine patients have sustained less serious injuries," the South East Coast Ambulance Service said in a statement.
The ambulance service said there were around 150 passengers on board the train which departed from London Victoria station and veered off the tracks shortly before 4:00 pm local time.
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."
Footage from the scene in East Sussex showed at least three overturned carriages and paramedics on site. The cause of the incident remained unknown.
The accident involved a train from Southern Rail, which connects areas south of London to the capital.
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.