LONDON - Croatia, Austria and Trinidad and Tobago have been added to Britain's quarantine list in an effort to contain the spread of coronavirus in the country, a senior British official announced Thursday.

British Transport Secretary Grant Shapps said that the restrictions come into force at 04:00 BST (0300 GMT) on Saturday.

Meanwhile, Shapps also announced that travellers arriving in Britain from Portugal will no longer need to self-isolate for 14 days.

He said the decision to change the travel corridors took a range of factors into account, including the estimated prevalence of COVID-19 in a country, the level and rate of change in the incidence of confirmed positive cases, and the extent of testing in a country.

The other factors included the testing regime and test positivity, the extent to which cases can be accounted for by a contained outbreak as opposed to more general transmission in the community, government actions and other relevant epidemiological information, he said.

The new move was announced after COVID-19 cases on the Caribbean islands more than doubled in a week.

The British Foreign Office said that it would look at putting any country with more than 20 cases per 100,000 people on its quarantine list.

Earlier this week, the British government refused to deny that tourists returning from Croatia and Greece could face quarantine measures after the number of cases per 100,000 people in Croatia reportedly rose above 20, seen as a key benchmark.

Downing Street said the government continues to "keep these rules under review" and that "protecting public health" remains the main priority.

Pushed on the limited time between new quarantine measures being announced and their implementation, Downing Street said: "It is important that when we make changes to the exemptions list, we do so in a swift way."

France, the Netherlands, Monaco, Malta and other countries were placed on the British quarantine list after 04:00 BST (0300 GMT) on Aug. 15.

Previously, Shapps announced that travellers returning to Britain from Belgium, the Bahamas and Andorra would have to quarantine at home for two weeks from Aug. 8, citing rising COVID-19 infection levels in the three countries.