The driver of a Eurostar train misdiagnosed a computer breakdown onboard, causing a nine-hour delay in the London-Paris trip at the weekend, the president of French SNCF rail operator said on Monday.

"The very long delay was caused by an error of diagnosis by the driver," said Guillaume Pepy, who admitted that SNCF bore "full and complete" responsibility for the nightmare journey of the 640 passengers.

Pepy said the driver had not "followed all the steps of the instructions of the repair manual to the end."

In an embarrassing new record for the flagship service — which normally takes 2 hours 15 minutes — 640 travellers spent 12 hours and 13 maddening minutes stuck on two trains after switching at Lille.

The passengers had left London on Friday evening and were due in Paris at around 11.30pm the same night. They arrived in the French capital shortly after 9am on Saturday.

Pepy said the train should have never left Lille and described the breakdown as an "extremely spectacular" but "very rare" incident.

AFP