Eurostar was investigating its latest embarrassing cross-channel train breakdown today which left more than 700 people stranded in darkness.
The Paris-to-London service ground to a halt with its lights off just south of Ashford, Kent, around 10.45pm yesterday, after what the operator described as a "major technical problem".
More than 740 passengers had to clamber down ladders on to the tracks and then back up on to a replacement train that arrived alongside more than an hour later.
Eurostar - which was heavily criticised for a series of pre-Christmas breakdowns that stranded thousands of passengers overnight - apologised for the latest stoppage and said a full investigation was under way.
A spokesman said: "We don't know exactly what happened or why at the moment. We are investigating fully."
She added: "We are sorry for any discomfort caused."
There were reports of stifling temperatures on board the stranded train, with toilets not working and staff struggling to find emergency lighting.
Passenger Richard Startari told the BBC: "Just 10 minutes from Ashford, the Eurostar kind of stopped, at which point we then lost all power.
"A lot of the Eurostar staff are going up and down with water, but they don't seem to have torches, which is a bit concerning. They're using their iPhones."
Another called Oliver told Sky News that conditions on board during the stoppage were "fine", adding: "It's getting fairly hot, but people are very calm, very relaxed."
Passengers were transferred coach by coach at around midnight, with everyone having to grab their own luggage.
The rescue train arrived at St Pancras just after 2.30am, more than four and a half hours later than the scheduled arrival time.
People were greeted with food and drink in the capital, where their mood was described by the operator as "extremely good". About 100 people also got off at Ashford.
Eurostar said travellers were offered taxis or hotel accommodation, and are being offered a cash refund of last night's Paris to London trip, plus a free return ticket in compensation.
A spokesman for the operator said: "The passengers feel they have been well informed and well looked after.
"It's not nice arriving at that time in the morning, but they understand."
The train was due to leave Paris Gare du Nord at 7.43pm UK time and arrive in St Pancras at 9.59pm, but the departure was delayed about an hour after an unattended bag caused a security alert.
Eurostar said it was not expecting any significant disruption to services between London and Paris as a result of the breakdown, and that passengers should check in as normal.
Today's 5.25am departure from St Pancras to Paris left on time, it added.
An independent report into Eurostar's pre-Christmas breakdowns - which left around 2,500 passengers stranded on trains for several hours overnight - criticised the preparedness of the firm for such an emergency.
Five trains broke down in the Channel Tunnel on the night of December 18 and 19 after snow got into the locomotives' power cars.
The report - published earlier this month - said the company had "no plan in place" to deal with the breakdowns, and said passengers, including expectant mothers and youngsters returning from Disneyland Paris, had to contend with "appalling conditions".
GNER East Coast Main Line rail boss Christopher Garnett and French transport expert Claude Gressier made 21 recommendations, including train modifications, improved communications and better emergency plans.
Eurostar also suffered more transport problems last week after a major Brussels commuter train crash on Monday.
Services to and from the Belgian capital have been disrupted since the collision.