Eurostar is again telling passengers only to travel from Paris to London “if absolutely necessary”.
Its services have been hit again by long queues caused by industrial action by French customs officers.
The officers are working to rule, and only one of the usual six X-ray machines is being operated.
The company, which has apologised, says tickets can be changed free of charge, or affected passengers can claim refunds.
A company spokesperson said that passengers who had to travel should turn up for the train they were booked on, but that they should not worry if they didn’t make it through the barriers on time, as they would be allowed on the next train departing.
It added that those who arrived at their destination more than 60 minutes late could claim compensation.
But Eurostar’s website is again recommending not to travel unless absolutely necessary.
Customs officers are demanding higher pay and better working conditions.
They also want more staff, which they say will be needed after Brexit to help process British citizens who will no longer have European Union passports.
The industrial action is due to last until 19 March.
Last week, French unions representing around 17,000 customs workers rejected a government offer of a €14m pay boost, saying it was not enough.