Thousands of holidaymakers are preparing to head overseas when the ban on foreign leisure travel is lifted in England and Wales on Monday.
Travel firms have reported a surge in demand for trips to Portugal, after the Government put the country on its green list for travel.
That means returning travellers will not need to self-isolate on their return, and are only required to take one post-arrival test.
EasyJet has added 105,000 extra seats to its flights serving green-tier destinations, while Tui will use aircraft which normally operate long-haul routes to accommodate the surge of people booked to fly to Portugal.
Only a dozen countries and territories are on the green list but most are either remote islands or do not currently allow UK tourists to enter.
The Government is advising people not to make non-essential trips to locations on its amber list, which covers popular destinations such as Spain, France, Italy and Greece.
But this guidance is expected to be ignored by some holidaymakers.
Those who do travel will be required to self-isolate at home for 10 days on their return, and take two post-arrival tests.
They can end their quarantine early if they receive a negative result from an additional test taken after five days.
Travel firms such as airlines and tour operators have called for quarantine and testing requirements to be relaxed, and for more destinations to be added to the green list.
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.
Scotland will permit foreign holidays from May 24.
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 the same date.