In a major blow to the UK travel industry, the British government removed Portugal from its safe travel list on Thursday.
It means anyone returning from the country will have to quarantine for ten days.
Britain also added seven more countries to its red list which require hotel quarantine.
Airlines and holiday companies have acted with fury and dismay.
But Transport Secretary Grant Shapps insisted the government was putting safety first.
"We've seen two things, really, which have caused concern. One is the positivity rate has nearly doubled since the last review in Portugal and the other is that there's a sort of 'Nepal mutation' of the so-called Indian variant, which has been detected. And we just don't know the potential for that to be a vaccine-defeating mutation..."
The government's move essentially shuts down the UK's travel market just weeks after it reopened.
Since May 17, the UK has been operating a traffic light system where travel to countries rated amber or red is not illegal but it is discouraged.
Portugal had been the only mainstream holiday destination on the green list.
It provided a lifeline to an embattled travel industry which now adds further cancellations and uncertainty to its woes.
Shares in the major airlines have tumbled and there have been calls for a financial bailout to save jobs.
For those British tourists already in Lisbon and Lagos, they are now puzzled about their fate back home.
"You have to have system where either it's open or it's closed. I mean, these families bringing out kids and there's people who have booked holidays for a long time."
"Yeah it is very frustrating. We came here because we like Portugal and it's in the green list that is fine so yeah it is going to impact my work and stuff when I get back."
"It is that constant changing of goal posts, that's the frustrating side of things."