All Eurostar trains have been cancelled because of smoke in one of the cross-Channel tunnels, the company said.
Eurostar said in a message posted on its website: "We are sorry but we are unable to run any further trains today because Eurotunnel has been closed due to smoke detected in the north tunnel.
"If you were planning to travel today, we advise you to postpone your journey and not to come to the station."
It said all trains were returning to original stations because the tunnel would be closed until further notice.
It comes as security remains heightened following the fatal terror attacks on the Charlie Hebdo magazine and a kosher deli in Paris last week.
Kent Police said the smoke was caused by a lorry fire.
A spokesman said: "A lorry fire has led to the closure of both bores of the Channel Tunnel.
"The fire was at the French end of the tunnel and is being dealt with by the French authorities. There are no reported injuries.
"However, rail passengers are advised to expect significant delays whilst the vehicle is being recovered and fumes are cleared from the tunnels."
Cross-Channel car service Le Shuttle tweeted: "CO2 detectors have been activated in one tunnel; we are currently awaiting feedback from teams in Tunnel. Further info to follow."
In March hundreds of Eurostar passengers were delayed after a lightning strike triggered a fire in a building close to the entrance to the tunnel in Kent.
Although there was no damage to the track, four trains in and out of England were affected. Three eventually reached their destination but the fourth, which had been heading to Paris, was forced to turn back to London.