Cross-channel traffic could be disrupted for several weeks after a fire and explosion on a freight shuttle seriously damaged part of the Channel Tunnel yesterday.

Thousands of passengers were stranded after all Eurostar trains between London and the Continent – including 10 trains already en route – were cancelled, with none expected today as fire-fighters continue to tackle the blaze.

The fire is believed to have started in the brakes of a lorry on a Calais-bound shuttle three miles from the tunnel exit. The flames rapidly spread to two other vehicles. Lorry drivers reported hearing an explosion but the French Interior Minister, Michèle Alliot-Marie, said the fire appeared to have been an accident.

French officials said terrorism, or sabotage, could probably be ruled out but the eastbound bore of the tunnel had been "seriously damaged". It might be several days before the undamaged bore of the tunnel could be safely reopened to trains, they said, and it might be weeks before the cross-Channel link was fully restored.

About 50 people, mostly lorry drivers, escaped without serious injury into the safety tunnel which runs between the twin, single-track tubes of the tunnel proper. French officials said six people had been affected by fumes and treated in hospital. Six others had suffered cuts or bruises.

There were no other trains on either track in the tunnel when the fire broke out at about 3pm British time.

The fire was the sixth of its kind since the tunnel opened in 1994 but the fight to quell it continued this morning after an operation involving hundreds of firefighters from Britain and France.

Huge queues of travellers booked on Europe-bound trains formed quickly at St Pancras. Cars and lorries bound for the Continent built up around Dover.

Those inside the shuttle, including the 14 injured, had to wait for several hours in the safety tunnel before they could be evacuated. Otherwise, the tunnel escape procedures operated perfectly, French officials said.

But thousands of Eurostar passengers were left stranded and confused, with many complaining about a lack of information and help from Eurostar staff.