In a UK-wide move, 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.
UK 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 Edinburgh University, suggested the decision may be “retaliation” in response to European countries requiring Britons to quarantine on arrival.
She told BBC Good Morning Scotland on Thursday the measure was “not necessarily” indicated 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,” she said.
While levels of the Delta variant in Portugal made the decision to some extent, Dr Tait-Burkard 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.”
Nicola Sturgeon denied any political motivation in the move and defended the traffic light travel system, which has been labelled “ridiculous” by some experts.
“It wasn’t political from my point of view, I can’t speak for anybody else, but no - [it’s] absolutely not,” she said.
“Where possible, given the geography of certainly England, Scotland and Wales, a four-nation approach makes sense.
“I’ve been clear in the past – if I don’t think the approach the UK Government is taking for England is the right one, then we will do something different in Scotland.
"It won’t be perfect, because people can come into the UK via England to Scotland, so that’s why four nations is best.
“We will make the case for the right four-nations approach, which we will continue to do, and where necessary take our own position.”