All rail services have been stopped since 2.57pm, when the alarm was raised on the French side.
Kent Fire and Rescue Service has responded to the alarm, and sent equipment to help deal with the blaze.
"We have a number of appliances there. It is a serious incident," a spokeswoman said.
Some 32 lorry drivers are known to have been evacuated from the train on which the fire broke out.
The fire is now said to be under control.
The tunnel has had a good safety record, with the last fire 10 years ago.
Passenger numbers have soard by 21% in the first quarter of 2008, thanks to the opening of St Pancras station and High Speed 1.