The Eurotunnel is "pretty much fully booked" for Friday as desperate Brits try to hurry home from France to beat quarantine.
And bosses warned that the Le Shuttle train hasn't "got the space to take everybody who might suddenly want to come up to the coast".
They warned that holidaymakers who don't have bookings not to travel to the terminal in Calais.
Moments after the UK government announced that all travellers returning from France would have to self isolate for 14 days the Eurotunnel website was inundated with new booking requests.
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At 10.35pm there were 3,563 people holding in a virtual queue to get tickets.
John Keefe, director of public affairs for Eurotunnel, told BBC Newsnight: “The thing we’re confronted with now is this sudden requirement for people wanting to travel back when they haven’t got a booking for tomorrow.
"We’re already pretty much fully booked tomorrow.
"This is peak season, summer holidays. People are returning on a Friday night to get back home and so we just haven’t got the space to take everybody who might suddenly want to come up to the coast.
"What we’re saying to people is amend your booking online. Make sure there’s space before you travel to the terminal.
"We’ve already got the highest capacity we can at peak times so by the time people get to us tomorrow afternoon we will be running a peak service.
"There’s some possibility of adding additional trains in the off-peak periods and we will try and do that where we can, but people must look online and check there is capacity there before they travel to the terminal."
The new rules for holidaymakers come into effect at 4am on Saturday.
The Netherlands, Monaco, Malta, Turks & Caicos and Aruba have also be added to the list of countries deemed unsafe.
France - the UK's second most popular holiday destination - was taken off the 'safe' list due to a rise in coronavirus infection rates.
There is estimated to be roughly 500,000 Brits currently in France.
Between August 6 - 12, new cases in France rose to 18.1 per 100,000, up from 13.1 for the previous seven days.