A number of Greek islands including Brit favourites Mykonos and Santorini have been put on the Eu's dark red list, which means that only essential travel is recommended.
It's yet another blow for holidaymakers hoping to get an overseas summer break, as coronavirus infections surge again thanks to more contagious variants.
The cluster of 13 South Aegean islands includes Greece's most popular destinations for foreign tourists which, combined, draw millions of people every summer.
Greece had relied on promoting "Covid-free" islands to draw visitors back this summer, hoping a rebound in international travel would resuscitate its vital tourism industry after its worst year in decades in 2020.
Despite a strong June in terms of arrivals, uncertainty remains over how the season will unfold.
"We're waiting to see how the (tourist) markets will react," said Manolis Markopoulos, president of the hoteliers association of Rhodes, where more than 90% of tourists are from abroad.
Germany and Britain are the biggest sources of visitors to Greece.
The dark red zones on the European Centre for Disease Prevention and Control's map help distinguish very high-risk areas and also helps EU member states uphold rules requiring testing on departure and quarantine upon return.
It comes as tens of thousands of holidays were put under threat amid fears Spain could be added to the amber-plus list.
Tourists returning from the Costas would still have to self-isolate even when the EU quarantine rule is axed.
Fears over the Beta variant could lead to experts recommending Spain is put on the list, which would dash the hopes of UK families hoping for a sunny summer getaway.
A July 14 paper from the Scientific Pandemic Influenza Group on Modelling, Operational sub-group, published today warned an increase in foreign travel is “of particular concern”.
The next shake-up of the traffic light regime is expected on Thursday.
A Whitehall source said senior No10 figures were split over whether to place Spain on the amber-plus list.
Transport Secretary Grant Shapps urged holidaymakers to wait until an announcement. He told the BBC: “I know it’s very difficult because people will be wanting to go away or have holidays booked or indeed be away.”
Mr Shapps said ministers would be able to clarify rules for Spain when they get details from the Joint Biosecurity Centre.
Spain recorded another 15,111 coronavirus cases on Thursday. And today it was reported the Beta variant in the country has fallen to 2.9% of Covid cases, down from 9%.
