The official said 32 people, all believed to be lorry drivers, were aboard the shuttle train where the fire broke out.
Twelve people were said to be suffering from smoke inhalation.
SNCF, the French rail operator, said rail traffic through the tunnel, which is 50.5km in length, would be suspended until Friday.
Estelle Youssouffa, Al Jazeera's correspondent in Paris, said: "A truck carrying liquids exploded inside the tunnel. French authorities say they have no information as to why the explosion happened. It is likely to cause major disruption... to passengers.
Michele Alliot-Marie, the French interior minister, was said to be travelling to the scene.
Fires have broken out in the past on the tunnel, which opened for commercial traffic in 1994, but they are rare.
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 the tunnel on November 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.