Thousands of Eurostar travellers were hit with delays today as traffic came to a standstill after a carbon dioxide alert in the tunnel.
Rail journeys were temporarily suspended between Britain and France after a detector went off shortly after 7am.
Eurostar, which runs high-speed trains through the tunnel between London, Paris and Brussels, said services should resume later today.
A Eurotunnel spokeswoman said: "A carbon dioxide detector went off in the tunnel and the normal procedure in these circumstances is to stop services and evacuate from trains.
"Thirty drivers on a freight train which was closest to where the detector went off were taken off the train and moved into the service tunnel."
The spokeswoman said a limited service had been started after one of the two tunnels reopened.
She went on: "The freight train was travelling towards England and was more than half way through the tunnel when it was stopped.
"Emergency services are trying to work out why the detector went off. We need to understand why it happened."
A Eurostar spokesman said: "We expect Eurotunnel to allow us to start services again later this morning.
"We are a standstill due to a problem that has been reported on a Eurotunnel shuttle train. But none of our trains are actually in the tunnel at the moment."
Kent Fire and Rescue Service said a train was halted after an alarm sounded.
A spokeswoman said crews were mobilised to the scene, inside the tunnel, at 7.17am but there was no fire.
She said crews were at the scene and were making sure the area was safe.
Nigel Shamber, duty inspector at Kent Police, said the incident was "just routine".
He added: "They have an awful lot of sensors in the tunnel and one of them went off. These things happen very frequently.
"There has not been a fire or any significant incident.
"Trains are now running in a reduced capacity."
Today's incident comes five months after passengers were caught in travel chaos when broken-down Eurostar trains were stuck for hours in the tunnel.