Eurotunnel said a CO2 sensor had triggered an alarm, setting off an emergency procedure in which all passengers have to be evacuated.

Eurostar, which runs high-speed trains through the tunnel between London and Paris and Brussels, said its services had been suspended for a period.

Lines have since started to reopen, but passengers were told to expect delays.

Dover Calais Ferries have plenty of spare capacity at present, after having coped very well with the additional passengers following air line disruptions caused by the volcanic ash cloud.

“The traffic is resuming now, we have received authorisation from Eurotunnel,” a Eurostar spokeswoman told AFP news agency. “But we expect a knock-on effect on the schedule.”

Services in both directions were halted when the alarm was triggered in the tunnel shortly after 0700 BST (0800 GMT).

The nearest train, a shuttle carrying 30 lorries and drivers, was evacuated and taken back to the surface on the UK side.

Emergency services from Kent were called.

Nigel Shamber, duty inspector at Kent Police, said: “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.”

A Eurotunnel spokeswoman said: “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.”

Nine Eurostar trains were delayed by between 50 minutes and three hours, French state rail operator SNCF said. Services leaving Paris, London and Brussels were all hit.

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