Travellers from London to Paris face 90-minute delays and train cancellations on the day of the Olympic Games opening ceremony.
Overnight, vandals damaged the high-speed line between Lille in northern France on what would be one of the busiest days of the year for Eurostar trains between the UK and French capitals.
Eighteen Eurostar trains are due to run between London and Paris, but an unknown number have been cancelled.
A spokesperson for Eurostar said: “Due to coordinated acts of malice in France, affecting the high speed line between Paris and Lille, all high speed trains going to and coming from Paris are being diverted via the classic line today Friday 26 July.
“This extends the journey time by around an hour and a half. Several trains have been cancelled.
“Eurostar’s teams are fully mobilised in stations, in the call centers, and onboard to ensure that all passengers are informed and can reach their destination.
“Customers are being informed via email, SMS and on Eurostar.com.”
Travellers booked on trains today are told online: “Due to a problem with the overhead power supply in France today, we advise you to postpone your trip as your train is likely to be delayed due to infrastructure issues.”
Passengers can get a refund or switch trains, but very few trains are available in the next 48 hours.
The disruption could not have come at a worse time for travellers, some of whom are planning to be in Paris for the Olympic opening ceremony.
It is impossible to switch to flights, because all three Paris airports will be closed from Friday afternoon as a security precaution.
Millions of French travellers will be affected. The French operator SNCF says: “Following this massive attack aimed at paralysing the high-speed line network, a large number of trains have been diverted or cancelled.
“Affected travellers will be contacted by email or SMS. We ask all travellers who can to postpone their trip and not to go to the station.
“All tickets for these disrupted journeys are exchangeable and refundable.”