The UK tourists will no longer need to quarantine after holidaying in Portugal, but travellers returning from Croatia will have to self-isolate.
Transport Secretary Grant Shapps said people will need to self-isolate for 14 days on returning from Croatia, Austria and Trinidad and Tobago.
The changes apply to anyone arriving after 04:00 BST on Saturday.
Meanwhile, the Scottish government has added Switzerland to its list of countries requiring quarantine.
The Portuguese government welcomed the changes as “useful for all those who travel between Portugal and the United Kingdom”.
It said the move was “proof of the good outcome of intense bilateral work” and “allowed for an understanding that the situation in the country has always been under control”.
But consumer group Which? said the change in rules for Portugal was “likely to come too late to help many struggling holiday companies” and called for support for the travel industry.
The latest updates to the quarantine list come after thousands of British holidaymakers made a last-minute dash to get home from France last weekend , before quarantine measures came into force.
It is thought around 20,000 British tourists are currently in Croatia.
