The fire occurred some 11 km from the French entrance to the tunnel at Calais at around 14:00 GMT on Thursday on a truck aboard a freight train carrying 32 lorry drivers and their vehicles.
No passenger trains were travelling through the tunnel at the time of the incident.
A spokesman for the French police said that the fire was thought to have started on a lorry that overturned. Media reports said the truck was carrying chemical phenol.
According to French radio, no one was killed, but 14 people suffered minor injuries, including smoke inhalation.
Traffic is expected to be halted in both directions until at least until Friday morning. The closure of the tunnel will affect thousands of travelers.
More than 4.6 million people traveled on Eurostar trains in the first half of 2008.
