More than 50 firefighters were called to the scene almost a mile inside a tunnel under the River Thames between Swanscombe, Kent and Thurrock in Essex.

The fire broke out on a train carrying cables involved in the construction of the new line linking Gravesend to St Pancras, London.

Another worker was taken to Darent Valley hospital with severe burns.

A spokeswoman for Kent Fire and Rescue Service confirmed a man had been found dead by four firefighters wearing breathing masks who were searching the train.

About 20 fire appliances were at the scene along with a water appliance and a foam appliance after the alarm was raised at about 1915 BST on Tuesday.

A Channel Tunnel Rail Link spokeswoman said: "The fire started in a maintenance locomotive in the Thames Tunnel which is part of the construction of section two of the rail link.

"I have not been told there was any sort of collision.

"Another train went in to get to the locomotive out, but I think the fire only involved one train."

The stretch of track, once it is completed, is not due to be used by Eurostar until 2007. The company has said it does not expect any current services to be disrupted.

An investigation into the blaze was beginning on Wednesday morning.