Passengers from the U.S. and the European Union have been arriving in the UK and will no longer need to quarantine.
The new rules, which came into effect at 3:00 a.m. local time on Monday, are only for those who have been double vaccinated against COVID-19.
Travelers still need to take a pre-departure lateral flow or PCR test for the virus and another on day two after arrival.
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France is excluded from the changes because of concerns about the Beta variant in the country.
Flights started arriving at UK airports early on Monday, including London's Heathrow.
Karen Tyler was there to welcome home her son Jonathan from Texas for the first time in nearly two years.
"I'm just so overwhelmed, I'm so happy," she said.
"FaceTime just isn't the same. He's just going to be spoilt so much for the next fortnight. I'm not going to let him go."
Ifti Khar Ahmed flew from Pakistan to the U.S. and returned home to London on Monday.
"[I'm] very happy, very happy. As you can imagine, coming home after such a long time is so exciting," he said.
Karen Tyler welcoming her son home to the UK after almost two years apart. /Reuters/ Peter Nicholls
Economic cost
Reuniting friends and family was one reason the UK government said it changed its guidance.
But pressure was growing on officials to further open up travel, given the widespread roll-out of vaccines in the country.
Official figures show more than 85 million jabs have been administered, with more than 88 percent of adults receiving one dose and 72 percent now fully vaccinated.
CEO of London's Heathrow Airport, John Holland-Kaye, said Monday's changes were "a good start" but that more help was needed to get the industry back on its feet.
"We are showing that the vaccine is our passport to freedom. We need to see the cost of testing come down and let's see the green list expanded. Let's be confident in the vaccines."
According to Heathrow, almost 957,000 passengers passed through the airport in June, down almost 90 percent from pre-pandemic levels.
It said the closure of the transatlantic route alone was costing the British economy around $32 million a day.
Traffic light system
The travel and tourism sector wants more countries added to the UK's "green list." In its traffic light system for travel, green destinations require no quarantine on arrival.
There are also concerns about a new "amber watchlist" being discussed by officials. Countries on it may move to the "red list" at short notice, which would mean people arriving from those places having to quarantine in a hotel for 10 days at a cost of $2,430.
The government's next review is due later this week but ministers have been cautious with changes in recent months.
It follows criticism over a failure to close borders to India earlier this year after the identification of the Delta variant there.
Last week, Transport Minister Grant Shapps told the BBC decisions would be based on "recommendations from scientists."