Britain abruptly imposed a two-week quarantine on all travellers arriving from Spain after a surge of coronavirus cases, a dramatic and sudden reversal on Saturday to the opening of the European continent to tourism after months of lockdown.
The quarantine requirement takes effect from midnight (2300 GMT on Saturday), making it almost impossible for travellers to avoid it by rushing home.
The British foreign ministry also advised against all but essential travel to mainland Spain, a move likely to prompt tour operators to cancel package holidays and trigger claims against insurers.
Europe's biggest travel company TUI said it was cancelling all departures on Sunday to Spain from the United Kingdom, and was urgently reviewing future flights.
"We're incredibly disappointed that we didn't get more notice of this announcement, or that this decision wasn't made yesterday, as many Brits travel on holiday at the weekend," said TUI's UK managing director, Andrew Flintham.
EasyJet and British Airways said they did not plan to cancel flights over the coming days.
Spain's Canary and Balearic Islands are not covered by the advice to avoid travel to the mainland, but holidaymakers returning to Britain from the islands will still be subject to quarantine on return.
Britain's government urged employers to be "understanding" towards staff who are unable to return to work for two weeks after coming back from holiday.
Transport minister Grant Shapps himself will be subject to quarantine, after a spokesman confirmed that he is currently on holiday in Spain.
The British move followed steps last week by other European countries. On Friday Norway said it would re-impose a 10-day quarantine requirement for people arriving from Spain, while France advised people not to travel to Spain's northeastern region of Catalonia.
But the total collapse of tourism from Britain would have far more impact. Britons made up more than 20% of foreign visitors to Spain last year, the largest group by nationality. Tourism normally accounts for 12% of Spain's economy.
Spain had been on a list of countries that the British government had said were safe for travellers to visit - meaning tourists returning home would not have to go into quarantine.
The announcement of such lists just weeks ago had allowed Europe's tourism sector to begin to revive after the near total shut-down prompted by the COVID-19 pandemic.
Spain is safe for tourists and Spaniards, the government insisted on Sunday after Britain abruptly imposed a two-week coronavirus quarantine on travelers returning from there, a decision that filled holidaymakers with dismay.
The UK move could deal a hard blow to efforts to restart the Mediterranean country's economy after months of lockdown.
"Spain is safe, it is safe for Spaniards, it is safe for tourists," Foreign Minister Arancha Gonzalez Laya told reporters.
The Spanish government will focus its efforts on trying to persuade Britain to exclude the Balearic and Canary islands from the quarantine measure, she said, adding that the prevalence of the virus in those popular travel destinations was much lower than in the United Kingdom.
Spain has seen COVID-19 cases rise in the last few weeks, and Britain announced late on Saturday it was taking the country off a safe-travel list. The quarantine took effect hours later.
The measure upset the plans of many people either on holiday or planning to take one and caused more disruption for airlines and tour companies.
While Gonzalez Laya avoided directly criticizing the UK move, British tourists traveling in Spain were more blunt.
"It's a bit crazy considering the restrictions in place in Spain already are really good, with the masks, with disinfecting everything, with hand wash in the shops. That's better than what we have in London," Rich Lambert, a communications officer, said at Barcelona's airport.
"This is ridiculous. There are almost no cases here and there's a far higher chance of contracting it when I get back to Britain," said John Snelling, 50, from Stratford-upon-Avon, who was on holiday in Menorca.