Portugal, which had been the only viable popular destination on the green list, will be moved to the amber list.
This will mean people returning from there will need to isolate at home for 10 days under coronavirus restrictions and take two PCR tests.
Other popular destinations such as Spain, Greece, Italy and France remain on the amber list.
Sri Lanka and Egypt are among seven countries moving to the red list, meaning returnees must quarantine at a government-approved hotel for 11 days at their own expense.
British travellers already in Portugal face a rush to return to the UK before the new rules begin in the early hours of Tuesday 8 June
One man told Reuters news agency: "You have to have a system where either it's open or it's closed.
news.sky.com
