Hundreds of seats on Eurostar trains from London to the continent are deliberately being left unsold so long queues don't build up at stations, the company has admitted.
A cut in the number of border officials means it takes nearly 30% longer to process passengers leaving St Pancras International than before Brexit and the pandemic.
Also contributing to the delay is the post-Brexit requirement to stamp UK passports for outbound travel.
Seat numbers are now being capped to prevent bottlenecks at stations.
The first daily services connecting London to Paris and Brussels can take up to 900 passengers.
But 350 of those seats are intentionally not being sold.
Eurostar runs the only high-speed trains that directly link the UK to France and Belgium via the Channel Tunnel.
As well as London's St Pancras, it also runs UK services from Ebbsfleet and Ashford, both in Kent.
Eurostar chief executive Gwendoline Cazenave said it was vital that the first trains to leave are on time to avoid a knock-on problem.
"If you delay the first train, then you delay the second and then it's a very bad customer experience," she said.
"Our customers say it's awful."
Passengers are being urged to arrive up to 90 minutes before their departure so they have sufficient time to get through border checks, which is three times longer than before the pandemic.