The shuttle train was carrying 32 people when the fire broke out just before 4 p.m. (1400 GMT), the spokesman said. Most were truck drivers accompanying their vehicles, and all were evacuated safely, he said.
The fire erupted on a single truck carried by the train, and traffic in the 30-mile tunnel will remain suspended until Friday, France's rail operator SNCF said. The cause of the fire had not been determined.
Eurostar said none of its trains were in the tunnel and its services had been canceled for the rest of Thursday.
The regional administration office in Calais, which is overseeing the response to the fire, said six people were injured in the fire.
Several people were sent to nearby hospitals for treatment, an office spokesman said, but he couldn't say whether they had suffered from smoke inhalation or other problems. Their conditions were not immediately available.
The Eurotunnel and regional administration officials were not authorized to be publicly named.
French Interior Minister Michele Alliot-Marie was traveling to the scene, her office said.
Fires have rarely broken out in the past in the tunnel, which opened for commercial traffic in 1994.
In August 2006, the tunnel was closed for several hours after a fire broke out on a truck loaded onto a freight train. No one was hurt.
A larger fire broke out aboard a train carrying heavy good vehicles through in the tunnel on Nov. 18, 1996. No one was killed but several people were injured and a large stretch of the tunnel was damaged. The fire led to new safety precautions for trains using the tunnel.
Associated Press writer Sylvie Corbet in Paris contributed to this report.