Eurostar services between London and the continent are returning to normal after thousands of travellers were delayed by a power fault in France.
Some passengers were delayed by up to nine hours and Eurostar has cancelled four trains scheduled for Tuesday.
It said a sagging overhead cable meant trains had to be switched from high-speed lines to slower ones, adding it was "very sorry for any inconvenience".
The train operator later said all other services were running normally.
The journey between London and Paris usually takes about two hours and 15 minutes.
Eurostar said it had cancelled two services from London to the French capital and two more heading back into St Pancras from Paris and Brussels respectively.
Passengers booked on cancelled services who considered their journeys "imperative" were offered alternatives later in the day, it added.
Eurostar said the disruption followed a fault with an overhead power cable owned by energy giant EDF, which was not available for comment.
Phil Piercey, from Walton-on-Thames in Surrey, said his holiday was "completely ruined" after it took him 11 hours to get home from Paris.
"There was absolute chaos at Gare du Nord and there was no information about possible delays. Eurostar staff were extremely unhelpful," he said.
"After an hour waiting at the station we were told our original train back to the UK had been cancelled due to hazardous high speed rail conditions."
He said he managed to catch the next train after a "mad scramble" but it eventually ground to a halt and was towed back towards Paris.
"We had no internal lighting, heating or communication from the staff on board. We were then told we had the choice of disembarking and returning to Paris or going on to St Pancras via Lille," he said.
"No other information was available. The whole journey was a complete nightmare.
"Passengers were only given bottles of water and a ration of shortbread biscuits. To leave people, including the elderly and those with young children, with no food for this length of time is outrageous."
Kandy Woodfield, from London, was on a train that left the city at 14:01 GMT on Monday - and was still on the train seven hours later.
"It has been hellish. We have not been kept informed," she said.
"They eventually gave us free water but that is it. The bar has made an absolute killing. I think it is terrible. There are elderly people and young children on board.
"I saw at least six policemen get on the train, I don't know if there has been any trouble but they keep walking up and down the train."
Similarly, Therese Kelly, from London, who was stuck for eight hours, said: "We had no announcements about why the train was so delayed or why it kept stopping, the train didn't move for hours.
"At one point policemen started walking through our carriage of the train, they wouldn't speak to us and I thought 'Oh God, are we in the middle of some security breach?' I was really scared. The whole train journey was brutal, totally brutal."
Six hours after leaving Paris Debbie Goodier, from Croydon, south London, had only reached Calais.
She had travelled to Paris for a day trip to celebrate her wedding anniversary.
"We were delayed two hours on the way out and now stationary on the way back. Not for the first time," she said.
Ryan Armstrong, who was travelling to Amsterdam via London, missed his connecting flight because of the delays.
"We were basically on the train for nine hours. We hardly moved; there was a lot of standing still. At one point the power was cut so we couldn't see anything or get any information," said the London man, who added that the staff on his train had been "helpful and supportive".
Some catwalk models and designers in the French capital for Paris Fashion Week were also caught up in the delays.
British model Poppy Delevingne told her 10,000 followers on Twitter she was "deliriously tired" after an eight-hour journey.
Fellow model Laura Bailey, a former face of Marks and Spencer, wrote that she had been stuck on a track for four hours.
And designer Henry Holland wrote simply "Eurostar disaster".
A Eurostar spokeswoman said journeys had been delayed by up to seven hours and four services were cancelled on Tuesday morning to "regulate" the service.
"We are trying to do everything we can to get passengers to their destination. We are doing everything we can to make their journey as comfortable as possible," she said.
Passengers with tickets for cancelled trains were told to exchange themfor another day if possible by calling 08432 186 186.