Demand for flights to Portugal this summer surged in recent days, suggesting that many British travellers expect the country to be on the government's green travel list.
On Friday, the government is expected to classify Portugal as a low-risk country to visit, because of its low rates of coronavirus infection, death and hospital admissions.
Airlines raised flight prices in line with demand.
British Airways is charging £530 ($736) for a flight from Heathrow to the Algarve on May 17, which is when the ban on foreign holidays will be lifted for people in England.
Flying the same route two days earlier costs only £234.
A Ryanair flight from Stansted to Portugal's capital Lisbon on the day overseas leisure travel restarts is £152, compared with £15 on May 16.
EasyJet is charging £234 for a flight from Luton to the Algarve on May 17, but only £73 the next day.
About 2.5 million British citizens visited the country in 2019.
The quarantine and testing requirements for people arriving in England when foreign holidays resume will be based on a traffic-light system, with destinations on green, amber and red lists.
People arriving from a green location will not need to enter quarantine and will have to take only one test after arriving.
Those returning from an amber country must isolate themselves for at least five days and take two tests.
The red list requires a 10-night stay in a quarantine hotel at a cost of £1,750 for solo travellers.
Assessments will be based on factors including the proportion of a country’s population that has been vaccinated, rates of infection, new variants, and its access to reliable scientific data and genomic sequencing.
There is speculation that this could lead to the green list featuring destinations such as Portugal, Malta, Gibraltar and Israel.
The latest weekly rate of new Covid-19 cases for every 100,000 people is 21.2 in the UK, 26.5 in Portugal and 28.6 in Malta.
On April 14, the Foreign, Commonwealth and Development Office stopped advising against all but essential travel to mainland Portugal.
The most popular European destinations such as Spain, Italy, France and Greece are initially expected to be on the amber list, but could switch to green before the summer holiday months.
Spain has a seven-day rate of 98.2 cases while Greece is on 113.9.
The UK's devolved governments have not announced plans to resume foreign holidays.
Tui, the UK’s largest holiday company, said it would offer customers coronavirus tests for a fraction of standard prices.
The cheapest package, aimed at people returning from green destinations, will be available for just £20, and consist of a lateral flow test and PCR test.
PCR tests alone typically cost £120, although several travel companies offer them for £60.
Tui said it is “subsidising the cost of testing to help customers travel again this summer”.
There are fears that testing requirements will make summer holidays unaffordable for many families by adding hundreds of pounds to the cost of a trip.
Rory Boland, editor of magazine Which? Travel, said the announcement of £20 packages is "great news – but only for Tui customers".
He urged the government to “reduce the cost of testing across the board, rather than have consumers rely on a system that is currently fragmented and flawed”.
More on UK travel
‘Perfectly legal’: UK citizens flouting hotel quarantine by flying to Turkey
England to give travellers a fortnight’s notice of changes to quarantine-free green list
Summer travel: expect testing and masks for some time to come
