A seat on a Ryanair flight from the capital Lisbon to Manchester on Monday costs £339, whereas the same route is just £75 on Wednesday.
Also British Airways is charging £348 for flights from Faro to Heathrow on Sunday and Monday, but the price drops to £137 on Tuesday.
The UK’s largest tour operator, Tui, said it has 9,500 customers in Portugal but that figure was already due to have fallen to 2,000 by Tuesday because of the end of half-term for schoolchildren.
The firm is allowing consumers to change dates, but is not offering refunds as the Foreign, Commonwealth and Development Office does not advise against travel to the country.
More on Covid-19
Communities Secretary Robert Jenrick said positive COVID-19 cases had doubled in the last three weeks in Portugal.
But Britons living and holidaying in the EU nation and others who planned to visit soon have spoken of their panic and devastation following the change in travel status.
Ms Dunne was due to come home from Portugal on Tuesday, and she has had to rebook for Sunday.
She told Sky News: “We panicked and booked a flight for tonight because we were so worried it would come in with immediate effect, and we can’t self isolate. We’ve managed to move our flights to Sunday but we’re still losing money on the hotel.
“It should just never have happened. The infection rate here is the same as the UK, especially in this part of Portugal. Why let us come in the first place if they were going to make us isolate?”
Michelle and Noel Brennan
The Brennans have a house in Portugal, and say they are “devastated” the country is going on to the amber list.
Michelle Brennan said: “I can’t believe we’re going through this again. It’s just like last year, total chaos. It’s just so sad.”
Mr Martins runs the Casa de Praia beach bar in Vilamoura.
He says 80% of his customers are British and “the decision to put Portugal on the amber list just doesn’t make sense, the numbers aren’t bad in this part of Portugal”.
He added: “Things were just getting back to normal, I don’t know how we’ll cope now without our British customers.”
Ms Teixeira, from London, had been due to be reunited with her family in Portugal for the first time in 18 months but says her hopes were “crushed” at the last minute by the government’s decision.