Earlier this week, UK Prime Minister Boris Johnson pointed out that the government will have "no hesitation" in moving countries off the so-called green list if necessary. The list includes nations that British holidaymakers can visit without having to go into quarantine after returning to the UK.

The British government has tightened the nation's borders and travel rules amid concerns over the emerging Nepali mutation of the Indian variant of COVID-19.

On Thursday, the Department for Transport (DfT) decided to move Portugal, including Madeira and the Azores, from green to amber status in order to "safeguard public health against variants of concern and protect our vaccine rollout".

Portugal being removed from the green list means that UK travellers will have to self-isolate for 10 days upon return from the country, which was the only mainstream tourist destination Britons could visit without having to quarantine.

Transport Secretary Grant Shapps described the move as "a difficult decision", adding that Portugal was downgraded because the UK government wants to give Britain "the best possible chance of unlocking domestically" on 21 June.

Shapps tied the decision to what he dubbed "a sort of Nepal mutation" of the Delta variant of the coronavirus that came to the UK from India. Public Health England (PHE) released data on Thursday, showing that the Delta variant currently accounts for at least 75% of COVID-19 infections in Britain.

"We just don't know the potential for that to be a vaccine-defeating mutation, and we don't want to take the risk as we come up to 21 June and the review of the fourth stage of the unlock", the transport secretary underlined.

Commenting on Portugal's removal from the green list, Shadow Home Secretary Nick Thomas-Symonds accused the government of causing chaos "with the mishandling of travel restrictions at the border".

"Labour has warned time and time again that this is leaving the door wide open to new strains of the virus. Moving Portugal onto the 'Amber List' is not the answer. The 'Amber List' itself should be scrapped", Thomas-Symonds asserted.

He was echoed by the British Airline Pilots Association (BALPA), which said in a statement on Thursday that "this excessive caution could be the final nail in the coffin for the travel industry which has borne the economic brunt of the COVID-19 crisis with no help from the government".

BALPA Acting General Secretary Brian Strutton, for his part, argued that the decision on Portugal "is a total disaster for the already fragile travel industry and is likely to lead to further airline failures and many more job losses".

"We understand that safety comes first, but with vaccination programmes going well in many countries, it seems the government is ignoring the evidence and is allowing safe countries to languish in the amber and red categories for no valid reason", he added.

Johan Lundgren, chief executive of the UK low-cost air carrier EasyJet, in turn, insisted that the decision "essentially cuts the UK off from the rest of the world".

The view was supported by Heathrow Airport CEO John Holland-Kaye, who said that "ministers spent last month hailing the restart of international travel only to close it down three weeks later, all but guaranteeing another lost summer for the travel sector".

Virginia Messina, senior vice president of the World Travel and Tourism Council, struck the same tone, arguing that dropping Portugal from the green list "will crush the confidence to travel, depress forward bookings, and deter holidaymakers". 

On Thursday, seven countries, including Afghanistan, Bahrain, Costa Rica, Egypt, Sri Lanka, Sudan, as well as Trinidad and Tobago, were added to the UK's "red list", which stipulates a mandatory 10-day quarantine in a hotel.

The countries still on Britain's green list include Israel, Iceland, Gibraltar, Australia, New Zealand, Brunei, Singapore, the Faroe Islands, and the Falkland Islands.