Cross-Channel rail operator Eurostar is to cancel services this week because of industrial action in France.
Services are being cancelled ahead of strikes in France
Six services have been cancelled for Thursday, and other trains may also be affected by disruption - passengers have been advised not to travel.
Friday services to Brussels will also be affected by strike action.
A spokeswoman for Eurostar said 25,000 people would be travelling on its services to Saturday's Rugby World Cup in Paris.
She added that 40% more passengers would use the cross-Channel rail service between midday on Thursday and Saturday afternoon, compared with the same time last year.
About 24,000 will travel back from France to the UK on Sunday and Monday, she added.
Industrial action is being taken by public sector workers in France on Thursday, the rail firm said.
Five services between London and Paris have been cancelled on Thursday, and also one from London to Brussels.
No Eurostar services will call at Calais that day, and there is a reduced service to Lille Europe station.
The rail company said there would be no services on the Paris urban transport network, including the Metro, on Thursday and "very limited" TGV services on the French high-speed network.
"Eurostar is therefore advising all travellers not to travel if at all possible," the company said.
On Friday, four Eurostar services are cancelled, one from Paris to London, and the others between London and Brussels.
The company has said Eurostar passengers will be able to exchange their tickets.