No injuries were reported.
Kent Police said the fire was at the French end of the tunnel.
Eurostar said rail passenger services between Britain and France were suspended after smoke was detected in the tunnel beneath the English channel.
Tunnel operator Eurotunnel said in a statement that a C02 alarm went off just before midday and that it was investigating the cause.
"If you were planning to travel today, we advise you to postpone your journey and not to come to the station," Eurostar said in a statement.