Thousands of people have departed on international flights after the ban on foreign holidays was lifted for people in Britain.
Travel firms welcomed the boost in demand but called for coronavirus restrictions to be eased for more destinations.
Portugal, one of just a handful of destinations on the Government’s quarantine-free green list, will welcome 16 flights from England at Faro Airport in the Algarve on Monday.
Tui is using aircraft normally reserved for long-haul routes to accommodate the surge of passengers.
But Sir Jeremy Farrar, director of The Wellcome Trust charity, warned there “is a risk” that the coronavirus variant first identified in India could be transmitted by people travelling out of the UK.
“I think travel should still be very cautious and only when absolutely essential,” he told BBC Radio 4’s Today programme.
This comes after Health Secretary Matt Hancock said on Times Radio that destinations on the amber list – which includes Spain, France, Italy and Greece – are “places that you shouldn’t go to unless you have an absolutely compelling reason”.
Those who do travel to an amber country are required to quarantine at home for 10 days on their return.
EasyJet chief executive Johan Lundgren said the rollout of coronavirus vaccines in the UK and Europe is “key to restarting travel in a safe way”.
He went on: “The latest data suggests that most of Europe could actually already from right now go on to that green list of categories.
“And we know there is huge pent-up demand for people to not only go on holidays but to visit friends and family that they haven’t been able to see because of the pandemic and the fact that it has been illegal to travel, so it’s a big day today for us.”
EasyJet has added 105,000 extra seats to its flights serving green-tier destinations this summer.
British Airways chief executive Sean Doyle said the resumption of overseas leisure travel is “an important milestone”, particularly for people who have been “separated from their families and loved ones for over a year”.
He added that the Government must “start progressively assigning green status to many more lower-risk countries as their vaccination rates increase”.
Heathrow boss John Holland-Kaye said he is “surprised and disappointed” that the Caribbean is not on the green list as “many islands pose such a low risk”.
He went on: “If you look at Barbados, its vaccination rollout is the same as Portugal, and its level of new cases is lower than the UK.
“You might say it embodies low risk.”
He added that “many companies will not make it to next year” unless the green list is expanded ahead of the peak travel months of July and August.
Retired couple Keith and Janice Tomsett, aged 72 and 71, from West Chiltington in West Sussex, departed on one of the first flights from Gatwick on Monday morning, travelling to the Portuguese island of Madeira.
Mr Tomsett said: “We’ve gone through all the hoops, PCR testing… after 15 months of being locked up this is unbelievably good.”
But he expressed frustration the pair will be required to take another test after they return to England.
“We’re fully vaccinated, we’re going to a green list country and we will be having a test 48 hours before we fly, so why we have to have another one two days later, I do not understand,” he commented.
The Government has pledged to update its lists on June 7, and will review its overall policy in relation to arriving travellers on June 28.
Non-essential travel from Northern Ireland to the Common Travel Area – which consists of the UK, Republic of Ireland, Channel Islands and Isle of Man, will be allowed from May 24.