The UK government has said it will review travel advice weekly, as it assesses guidelines for holidays on a rolling basis due to the coronavirus. Previously, this was reviewed every three weeks.
Currently, Portugal is off the “air bridge” list and is one of the countries not exempted from quarantine rules.
The new timeframe for reviewing travel advice means air bridges could collapse at short notice, leaving travellers facing quarantine on return to the UK.
In changes that apply to England, travellers from Estonia, Latvia, Slovakia, Slovenia and St Vincent and the Grenadines now won't have to isolate.
There are currently 80 nations that do not face travel restrictions when coming into England.
The Portuguese government has expressed regret at the decision not to allow tourists without quarantine, branding it “absurd.”
Portugal's Ministry of State and Foreign Affairs said: "It is a decision that is neither substantiated nor supported by the facts."
Portugal is one of the top destinations for UK tourists, with 2.1 million travelling there last year.
Spain is still on the list of countries exempted from quarantine rules, despite a recent spike in cases.
On Tuesday, 2,615 new cases were recorded in the country for the last week, which is almost 20 times the daily average recorded last month.
Paul Charles, chief executive of PC Agency, a travel consultant, told The Times: “Portugal is one of the most popular destinations for the British to visit in August. This decision is surprising bearing in mind their case numbers are declining, and tourists can easily drive in to Portugal from Spain.
“With hundreds of thousands of British travellers set to go to Portugal in the coming weeks, the irony is that the corridors which were meant to create so much certainty are now destroying confidence among consumers.”