The British government announced on Twitter late on Thursday night that it would be imposing a quarantine on all arrivals from France, starting at 4am on Saturday.

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The announcement prompted a rush of holidaying Brits, desperate to get home before the new quarantine measures kicked in.

British workers are not entitled to sick pay if they are quarantining, so people unable to work from home face the prospect of not being able to work for a fortnight under the quarantine rule.

Now transport operators have warned people not to turn up at French ports, stations or airports unless they have a valid ticket for travel on Friday, as services are extremely busy.

Eurostar tweeted: “Our trains to the UK are very busy today. To ensure we're able to help you get home if you need to, please only come to the station if you have a valid ticket for travel today.”

While P&O Ferries added that their service from France and the Netherlands – which is also the subject of a UK quarantine from 4am on Saturday – are also extremely busy.

Thousands of people had also headed to Calais, hoping to get the Channel Tunnel back, but the tunnel operator also advised that all travellers – including people with a FlexiPlus ticket who can normally turn up without a reserved crossing – must alter their ticket time online before showing up at the terminal.

Claudia, a 42-year-old German who lives in London but is currently on holiday in southwest France, called it an “absolute nightmare”.

“Even if we wanted to we could not come back in time. Eurotunnel is sold out for any slot after midday,” she told AFP.

And even those who did manage to make it back faced hefty charges for last-minute changes to their travel plans.

French student Antoine, 23, had to rush back to Bristol, where he is at university, cutting short his summer holidays.

“I'm a waiter in a small café near college, I can't afford to spend 14 days in the house,” he told AFP at London's St Pancras railway station after getting off a Eurostar train.

“I was supposed to come back on Monday morning, but as soon as I heard the news I rushed to the Eurostar app to change my ticket – €125 more than my original ticket.”

Although the official response from the French government said only that it “regretted” the UK's decision – which it intends to reciprocate – other French politicians were not so restrained.

Alexandre Holroyd, the French MP who represents French citizens living in the UK, called the quarantine “another incomprehensible and unilateral decision by Boris Johnson that has thousands of families on both sides of the Channel in anguish”.

While Nathalie Loiseau, MEP and France's former Europe Minister, called it “a decision with no scientific basis which will further complicate relations between the French and British citizens, already shaken by the Brexit . . . British exceptionalism is decidedly misguided.”