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.
Responding to the changes, Which? Travel editor Rory Boland said the government had "now made it clear that countries can be removed or added from the travel corridor list at a moment's notice".
He said the policy made it "too risky" for those who are unable to quarantine to travel.
But he added that holidaymakers who want to follow government advice and avoid non-essential travel to specified countries are finding it "increasingly difficult to claim a refund".
Mr Boland also called on the government to provide "urgent" support to the travel industry, adding: "The addition of Portugal is likely to come too late to help many struggling holiday companies who are at the point of collapse, as summer trips have already been cancelled."
The Tucker family, from Cambridge, were at a waterfront café on the Croatian island of Solta, off the coast of Split, when they heard they would have to quarantine on their return to the UK.
"We already cancelled a holiday in Barcelona because of quarantine rules," said mum Luzita, 50, a childminder.
"We've always wanted to come to Croatia so we looked at the infection rates and they seemed very low."
She said it was good the government had acted decisively, but suggested there were other options.
"Why not [carry out] virus testing at the airport when we arrive back in the UK? And surely using public transport to get home could be a risk."
Diane Barwick was in the Croatian town of Zadar visiting her daughter - having cancelled a planned trip to France when that country was removed from the exemption list.
She told BBC News: "My daughter should have been married here in May. I've not seen her for nearly a year and have had three flights cancelled this year."
Unlike many other British visitors to Croatia, she had responded to rumours that the country was set to be removed from the exemption list by booking an alternative early flight home.
That means she should be able to get home before the deadline and back to her job in retail.
"If you're in France you can get the boat or Eurostar. Here it's a flight only. I've got to travel three hours tomorrow to get to the airport in Croatia," she said.
The Department of Transport has advised people in Croatia, Trinidad and Tobago and Austria to follow local rules and check the Foreign Office website for further information.
In a statement, it urged employers to be "understanding of those returning from these destinations who now will need to self-isolate".
But children currently on holiday in those three countries will now miss the start of the new school term in England, Wales and Northern Ireland - unless their parents can get them home before 04:00 BST on Saturday.
People who do not self-isolate when required can be fined up to £1,000 in England, Wales, and Northern Ireland. In Scotland the fine is £480, and up to £5,000 for persistent offenders.
BBC Balkans correspondent Guy De Launey said only a small number of direct flights from Croatia were due to reach the UK before the deadline of 04:00 BST on Saturday.
The UK introduced the compulsory 14-day quarantine for arrivals from overseas in early June.
But the following month, the four UK nations unveiled lists of "travel corridors", detailing countries that were exempt from the rule.
Since then it has periodically updated that list, adding and removing countries based on their coronavirus infection rates and how they compare with the UK's.
In July, the Portuguese government expressed "regret" at the UK's decision to continue to exclude it from the safe travel list.
The country's foreign minister had previously said he hoped an "air bridge" between the UK and Portugal could be secured by the end of June.
The UK provides the largest number of overseas tourists to Portugal, with more than two million tourists visiting every year.
The Algarve coast is the most popular destination, with 1.2 million visitors from the UK last year.
Travel expert Simon Calder tweeted that the cost of flights from Manchester to Faro on Saturday morning had risen from £50 to £98 in 30 minutes.
"A good time to book that late summer break, though fares are already soaring," he said.
According to the Department for Transport, weekly coronavirus cases are on the rise in Croatia, Austria, Trinidad and Tobago as follows:
* Croatia - 164% increase
* Trinidad and Tobago - 232% increase
* Austria - 93% increase
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