Starting Saturday, travelers entering British territory from the three newly-added countries will be required to self-quarantine for two weeks.
The sharp changes in UK quarantine regulations were announced on Twitter by Transport Minister Grant Shapps.
The inclusion of the three countries is the latest in a series of adjusted quarantine policies Britain has undertaken to prevent the import of new coronavirus infections as Covid-19 hotspots emerge overseas.
"Data ... shows we can now add Portugal to those countries INCLUDED in Travel Corridors," he said.
Shapps said Britain's whole travel policy was constantly under review and could change quickly, warning that people should only travel if they were content to quarantine themselves should the rules change during their holiday.
Emphasizing that point, he also announced Croatia, Austria and Trinidad & Tobago were being added to the quarantine list, and incoming passengers would need to self-isolate for 14-days after they arrive, starting from 3am GMT on Saturday.
Portugal, a popular tourist destination for Britons, has previously expressed frustration that Britain had refused to drop its quarantine requirements, something which has hit the tourism-dependent economy hard.