People arriving in Wales from Portugal must self-isolate for 14 days from Friday, but government advice covering England remains unchanged.
Scotland is also expected to impose quarantine restrictions on travellers from Portugal this weekend.
Cases in Portugal have risen in the past week beyond the threshold at which ministers generally consider imposing 14-day mandatory self-isolation.
One travel expert said the situation left consumers “totally confused”.
The quarantine rules introduced in Wales apply to travellers from Portugal, Gibraltar, six Greek islands and French Polynesia, and come into force at 04:00 BST on Friday.
The six islands are Crete, Mykonos, Zakynthos (or Zante), Lesvos, Paros and Antiparos.
Changes to the rules for arrivals from Greece coming to England were also considered by ministers – but Greece will stay on the safe list for now.
Scotland has already reintroduced self-isolation measures for arrivals from Greece.
UK Transport Secretary Grant Shapps said on Thursday: “There are no English additions or removals today.
“We continue to keep the travel corridor list under constant review and won’t hesitate to remove countries if needed,” he added.
“Nonetheless, holidaymakers are reminded – 14-day quarantine countries can and do change at very short notice.”
He said the government takes several factors into account, including the prevalence of the virus as well as the level and rate of change, how many tests the country is doing, the extent of the contained outbreak and the government’s actions.
In Portugal, the seven-day infection rate has increased from 15.3 to 23 per 100,000 people.
A seven-day rate of 20 is the threshold above which the UK government generally considers triggering quarantine conditions.
In Greece, overall, there were 13.8 coronavirus cases per 100,000 people in the seven days to 2 September, down from 14.9 a week earlier.
But the changes have drawn criticism from industry experts as well as holidaymakers.
Paul Charles, chief executive of travel consultancy firm The PC Agency said: “The quarantine policy is in tatters and dividing the United Kingdom.
“Consumers are totally confused by the different approaches and it’s impossible to understand the government’s own criteria any more on when to add or remove a country.
“The current strategy has to change. The weekly reviews have been causing anxiety and financial pain for so many consumers and travel firms,” he added.
What are the latest quarantine changes?
England and Northern Ireland: No further additions – latest lists can be found here
Wales: Quarantine for travellers from mainland Portugal, Gibraltar, French Polynesia and six Greek islands – Crete, Mykonos, Zakynthos, Lesvos, Paros and Antiparos.
Earlier, Britons in Portugal said they feared the quarantine rules may change while they were on holiday.
Laura McDonald, 26, who is with her fiancé Jake Robson, 27, in the Algarve, told BBC News it would cost them £450 to get home early – despite originally paying just £230 in total for their return flights.
“It’s just not feasible. What we would pay is half of what we lose in wages each,” she said.
Ms McDonald said the resort of Albufeira already seems quieter following speculation earlier this week about the possibility of having to quarantine on arrival back in the UK.
“We were on the beach yesterday and there were a lot of British people there talking about it and reading the news,” she said.
“Whilst we are having a great time it is at the back of our mind and we are anxiously awaiting the next announcement,” she added.
Meanwhile Joanne Dayer, 55, from Worcester, said the prospect of having to quarantine felt like “a major inconvenience and a frustration that seems unnecessary”.
The NHS physiotherapist is currently in Faro with her husband and daughter. She said she has already had the virus, adding that she is “not so much concerned about the virus as the need to quarantine”.
“My husband is retired, and my daughter is going to university in October, but I will have to cancel all my patients,” she said.
Some holidaymakers have told the BBC they have paid as much as £1,000 for flights amid a scramble to get home from Portugal this week.
Every year, more than two million Britons visit Portugal, making up the largest number of overseas tourists to the country.
Most head to the Algarve in the south, drawn by sunny Atlantic beaches, picturesque fishing villages and golf courses.
During May and June, the Portuguese government reopened its restaurants, coffee shops, museums and beaches. Hotels have mainly reopened, but nightclubs remain closed.
Holiday giant Tui temporarily ended package tours to the Zakynthos resort of Laganas following a number of cases linked to the town.
Greece has insisted it is doing “everything in our power” to keep UK holidaymakers safe.
Last week, the UK government imposed a mandatory quarantine for arrivals from Switzerland, Jamaica and Czech Republic – but removed the requirement for arrivals from Cuba.
Are you currently on holiday in Portugal? Have you made plans to travel there? Share your thoughts by emailing [email protected].
Please include a contact number if you are willing to speak to a BBC journalist. You can also get in touch in the following ways:
- WhatsApp: +44 7756 165803
- Tweet: @BBC_HaveYourSay
If you are reading this page and can’t see the form you will need to visit the mobile version of the BBC website to submit your question or comment or you can email us at [email protected]. Please include your name, age and location with any submission.