Thirty-nine flights are scheduled to depart from Faro Airport in the Algarve for the UK on Monday, nearly double the usual total.
The final departure of the day is a Wizz Air flight due to land at Doncaster Sheffield Airport at 2.05am on Thursday.
Travellers must arrive in the UK before 4am on Tuesday or they will be required to self-isolate at home for 10 days.
This is due to the Government’s controversial decision to remove Portugal from its green travel list.
Tourists wanting to beat the deadline are being hit by a combination of many flights being sold out, and the handful of available seats being sold at inflated prices.
Ryanair is charging £285 for a flight from Faro to Bournemouth on Monday, but just £17 on Wednesday.
EasyJet flights from Faro to Gatwick are £227 on Monday and £53 on Tuesday.
A seat on British Airways’ last flight on Monday, from Faro to Heathrow, costs £349, although the airline is allowing people booked on flights from Portugal to the UK between Tuesday and Sunday to travel on Monday for no extra charge.
Tourists have also reported difficulties obtaining the pre-departure tests required by the Government for people arriving in the UK.
Many holidaymakers and travel firms expressed anger when the announcement on Portugal was made last Thursday, as it came just 17 days after the ban on international leisure travel was lifted.
Alan and Lisa Pechey, from Cambridge, who were on holiday in Lisbon, paid a total of £800 to fly back to Gatwick on Monday, earlier than planned.
Mrs Pechey, 66, told the PA news agency: “It was really expensive and I think the Government was totally unfair to throw that at us on Thursday because it really spoiled our holiday, totally.
“We had flown out on Monday for a relaxing break, but from Thursday onwards we were under extreme stress.”
Ana Pacheco, 28, from Islington, north London, who was on holiday near Porto, paid £300 for her flight home.
She said: “I lost money on this trip, about £300 extra, because I was due to come back tomorrow evening, so it is quite annoying.
“I think there should have been extra time added on for us to get home – at least a week would have been better.”
The Department for Transport said the situation in Portugal “required swift action to protect the gains made with the vaccine rollout”.
It stated that the positivity rate for coronavirus tests in Portugal had nearly doubled since the travel lists were first created four weeks earlier.
The DfT added that 68 cases of the Indian mutation, which is also known as the Delta variant, have been identified in Portugal.
Separate Test and Trace figures show 200 arrivals from Portugal were tested between May 6 and May 19.
Three of those people tested positive for coronavirus.
- Reporting by Press Association