Eurostar services to and from London St Pancras International have been cancelled due to flooding in a tunnel, as festive disruption continues.
At least 29 services have been cancelled so far. Eurostar said it was "working hard" to ensure later trains could run, but warned of severe delays.
Southeastern's high-speed services to Ebbsfleet have also been cancelled until the end of the day.
Travel journalist Simon Calder said St Pancras was "in chaos".
People with New Year trips to Disneyland were in tears, he said, because their services were cancelled.
Eurostar, which runs services from London to Paris, Brussels and Amsterdam, said affected customers were entitled to a full refund or exchange.
At around 11:30 GMT, the operator said it had cancelled further trains up until at least 16:12 due to the flooding "not improving".
"Network Rail is continuing to do all they can and will update Eurostar if any services can continue later in the day," a spokeswoman said, adding Eurostar was "extremely sorry" for the situation.
HS1, which operates the track, said engineers had worked through the night to remove water but the volume of water was "unprecedented".
Pumps and tankers are on the site and water levels are reducing, a spokeswoman added.
"We understand how frustrating this is for passengers and apologise for the inconvenience caused at such an important time of the year," she said.
Southeastern said no services would run from St Pancras, Stratford International or Ebbsfleet International.
It is the second time in 10 days there has been major disruption to Eurostar services with a "last-minute strike" by French workers halting trains before Christmas.
Daniel and Ramona, who were travelling to Paris for New Year's Eve, said they were trying to stay optimistic that their train would be "one of the lucky ones" to still run.
"[It is] very, very busy, not a lot of information being given out, a lot of people who don't know what to do with themselves, a lot of people sitting down and waiting - seems like they are in for the long haul," Daniel said.
"We'll have to just wait and see who goes and who doesn't."
Brian, who was travelling to Paris with his family, said: "We've been here in London for five days and we're supposed to be four days in Paris so we're trying to figure out how to get over there."
Cindy, who was returning home to the Netherlands, said her family were in good spirits but they hoped it would be fixed "or we won't be going home".
Separately, there is major disruption on Thameslink services - which run through London and across the south-east of England - and a reduced service on Northern Rail, which covers the north of England, due to staff shortages.
In Wales, the line between Llandudno Junction and Blaenau Ffestiniog has been closed due to "severe weather", while trains between Swansea and Shrewsbury have been cancelled due to flooding.
The Met Office has issued yellow warnings for wind, rain and snow for large parts of the UK.
Speed restrictions are in place on the West Highland Line until 09:00 on Sunday due to the forecast, Network Rail Scotland said.
Some flights at Glasgow Airport were also delayed due to snow on the runway.
Yellow weather warnings are in place for:
Rain across most of Wales until 18:00
Rain and snow for most of Scotland until midnight
Wind across Wales, the south of England and parts of the East Anglian coast until 03:00 on Sunday
Forecasters say flooding of homes and businesses is possible, as well as interruption to power supplies.
It comes days after Storm Gerrit left thousands of homes without power and caused disruption across the country.
Gusts of between 45 and 50mph (80.5km/h) have been forecast widely, while coastal areas in the west and south of England may see the strongest gusts of between 65 and 75mph.
Earlier this week, Storm Gerrit brought widespread disruption across Scotland and northern parts of the UK with a localised tornado tearing of roofs in Greater Manchester.
