Railway company Eurostar has launched an investigation after passengers on one of its services took 11 hours to travel from London to Paris.
More than 600 passengers who left London on Friday evening arrived in Paris on Saturday morning after technical problems caused delays.
The cross-Channel journey should have taken two hours.
Eurostar apologised for the "completely unacceptable experience" and said it had launched an internal inquiry.
Working order
When the passengers boarded the 2105 BST service to Paris, their train was in working order.
But another train travelling in the opposite direction had got as far as Lille when a warning light came on, which meant it could not satisfy the Channel Tunnel safety requirements, or travel through to Kent.
"We apologise profusely to our travellers for the unacceptable delay, inconvenience and frustration"
Both services stopped at Lille, where the passengers swapped trains at about 2300 French time.
The Paris-bound travellers spent 90 minutes waiting in Lille station.
On their second train, they continued their journey, but at 0100 French time, and 120km (74.5 miles) north of Paris, the train broke down, losing power completely.
It took three hours for them to be moved to a third train and, because the broken-down train was stuck, the passengers walked along the trackside.
After getting onto the third train passengers finally arrived into Paris on Saturday at 0910 local time, instead of the expected arrival time of 2326 on Friday.
'Terrible night'
One passenger complained that people had begun to smoke onboard one of the trains as they waited for it to move.
"Pretty quickly the atmosphere became stuffy and it's really not bearable," he said.
Mireille Faugere, a spokeswoman for French rail operator SNCF, said the 640 passengers had a "terrible night".
"For us, this situation is totally unacceptable and I've ordered an inquiry to know exactly what happened," she said.
"We were really unlucky because there was a whole series of technical problems causing a snowball effect, which is the reason why there was so much delay."
A Eurostar spokesman said: "We apologise profusely to our travellers for the unacceptable delay, inconvenience and frustration that they have suffered.
"We are offering everyone on board a full cash refund on both legs of their return tickets and in addition a free return ticket for future use."
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