Fully vaccinated passengers returning to England, Scotland and Northern Ireland from France will no longer need to quarantine after Sunday.
Under widespread changes to the traffic light system for travel, France is being moved from amber-plus to amber.
It was added to the list last month amid concerns about the Covid Beta variant, which scientists believe may be more resistant to vaccines.
But thousands of Britons are in Mexico, which is moving to the red list.
It is one of four countries now considered to be among the highest risk destinations.
Despite prior speculation, Spain will remain on the amber list, enabling travellers who are fully vaccinated to continue to enjoy a quarantine-free return.
However, the Department for Transport has now said that "arrivals from Spain and all its islands are advised to use a PCR test as their pre-departure test wherever possible" instead of the cheaper lateral flow tests.
But in a series of tweets Conservative MP Huw Merriman, chairman of the Commons Transport Select Committee, criticised "expensive" PCR tests for travel as an "unnecessary rip-off" and a "barrier to affordable travel".
India, Bahrain, Qatar and the United Arab Emirates are being moved from the red list to the amber list as part of the changes, which come into effect from 04:00 BST on 8 August.
And Germany, Austria and Norway are among seven nations being added to the green list.
The total number of countries or territories on the green list - from which all travellers can return without having to quarantine - will rise from 29 to 36.
But other countries have their own rules about allowing visitors - so being on the UK's green list does not guarantee travellers can visit there.
There are no changes to the rules requiring travellers to take tests before and after their return.
Transport Secretary Grant Shapps said: "We are committed to opening up international travel safely, taking advantage of the gains we've made through our successful vaccination programme, helping connect families, friends and businesses around the world."
Asked about the advice on PCR tests for arrivals from Spain, he told BBC Radio 4's Today programme that ministers were "not asking people to do anything different from what's been happening in Spain".
He insisted the government was "not changing the rules", as in Spain's case, "virtually everyone is taking a PCR" to meet the government's performance standards for the pre-departure test - adding that PCR tests helped scientists monitor variants.
He said that between 5,000 and 6,000 British holidaymakers are in Mexico currently and would be "making arrangements to come home". They need to return by the 04:00 BST deadline on Sunday or pay for hotel quarantine.
At-a-glance: The latest changes
Moving from amber to green: Austria, Germany, Slovenia, Slovakia, Latvia, Romania and Norway
Moving from red to amber: India, Bahrain, Qatar and the UAE
Moving from amber to red: Georgia, Mexico, La Reunion and Mayotte
Moving from amber-plus to amber: France
If you have been in a red list country in the last 10 days, you can only enter the UK if you are a UK or Irish national or UK resident, and you then have to stay at a quarantine hotel.
The government has announced the cost of staying at one of those hotels is increasing, with the price for single adult travellers rising from £1,750 to £2,285 from 12 August, with a second adult paying £1,430.
This better reflects the costs involved, the government has said. That includes transport to the hotel, security, provision of welfare services and the two PCR tests which must be taken on day two and day eight of the stay.
Children aged 5-12 will still cost £325; it is free for children aged under five.
British Airways said its teams had been "working through the night to arrange as many additional seats out of Mexico as possible to help get Britons home" before the new rules kick in.
It added that British Airways and British Airways Holidays customers could re-book their flights from Mexico "at no additional cost" ahead of the deadline, and that British Airways Holidays would "automatically refund any customer with a booking to Mexico in the next four weeks".
The UK government sets the red, amber and green lists for England, while the other nations are in charge of their own lists. Scotland and Northern Ireland confirmed they will be adopting the same changes as England.
But the Welsh government criticised the "ad-hoc nature" of the UK government's travel decisions. It said it will consider whether to follow the latest changes, adding: "We continue to advise against all but essential travel abroad because of the continuing risk of infection."
Is this enough to save the summer season for the travel industry?
There are some things for the sector to be pleased about. The green list is longer, France is fully amber and there are more countries turning amber from red.
But there are stings in the tail too. Of the seven green countries, only two - Latvia and Slovenia - currently allow in non-vaccinated UK tourists without quarantine.
Many major holiday destinations like Greece and Spain are still amber. And the testing regime - which many in the industry want scrapped - is still firmly in place.
The government says it is being cautious and continuing to protect the UK from dangerous variants, and that this is a good step for passengers and travel.
But after months of changes and uncertainty, there are concerns in the travel sector that this doesn't do enough to reassure the public to book.
British couple Katherine and Henry Walker, who own a campsite in west France, said they hoped the news will bring a flurry of last-minute bookings, but added: "I think it's too late for families to come because they would have booked elsewhere in the UK."
They said they were at 40% occupancy - when they would usually be at 90% - because of the lack of visitors from the UK.
Labour's shadow transport secretary Jim McMahon criticised the government for its "flip-flopping over France" and said it needed to explain how it had reached decisions.
"Ministers need to get a grip and set out a proper strategy, provide full data, and progress work with global partners on international vaccine passports so travellers and the industry can have clarity instead of reckless U-turns and confusion," he said.
British Airways boss Sean Doyle welcomed the news but urged the government to go further, saying the UK's economic recovery "is reliant on a thriving travel sector and right now we're lagging behind Europe, with our more stringent testing requirements and a red list significantly broader than our European peers".
James Martin, director of policy at the British Chambers of Commerce, urged the government to "fundamentally simplify" the traffic light system to give businesses "confidence and clarity".
He also called on the government to "step up efforts to drive down the cost of tests" as cost remained a "significant barrier to both business and leisure travellers".
And Tim Alderslade, chief executive of the industry body Airlines UK, said the announcement was "another missed opportunity", with UK travel opening up "far slower" than the rest of Europe.
Are you in Mexico? Are you planning on returning before Sunday to avoid hotel quarantine?
Or are you returning from Spain with a PCR test?
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