Portugal will be moved from the green to amber travel list, requiring visitors and returning holidaymakers to quarantine in the UK from 4am on Tuesday.
The UK’s Transport Secretary Grant Shapps blamed a rising infection rate and the apparent detection of a “Nepal mutation”, increasing concerns about the risk level.
But Dr Christine Tait-Burkard, an expert in infection and immunity at the University of Edinburgh, suggested the decision may be “retaliation” in response to European countries requiring Britons to quarantine on arrival.
Asked whether it was a necessary measure, Dr Tait-Burkard said: “Not necessarily in terms of case levels.
“Portugal is still very similar to the UK actually, and so that means the actual risk of going to the country is not necessarily higher than staying in the UK.
“What has made the decision to some extent is that Portugal now also has the Delta variant.
“We should probably not be surprised, given that British tourists went there and we have a large number of Delta variant cases.
“There’s also concern about a so-called Nepal variant which is kind of a mix of the Delta variants and the Kent (Alpha) variant.
“That is causing some concern, but it is a very last-minute announcement again.”
Speaking on the BBC Good Morning Scotland programme, she added: “It potentially is also a little bit politically motivated, seeing that many European countries have put the UK on the quarantine list.
“It might be a little bit of political retaliation behind that as well.”
Sri Lanka, Egypt and five other countries will also be added to the red list, requiring isolation in a Government-approved hotel, it was announced on Thursday afternoon.
The news came on the same day the Government said a further 5,274 lab-confirmed cases of coronavirus had been recorded in the UK as of 9am on Thursday, the highest single-day figure since March 26.
Portugal’s seven-day rate of coronavirus cases per 100,000 people stands at 37.2, up from 30.7 a week earlier.
That figure in the UK is currently 28.6.