Jet2 is contacting customers on the Balearic and Canary Islands to ask them to end their package holidays early, the BBC has learned.
Hundreds of customers have had flights back to the UK cancelled and been asked to leave sooner.
Jet2 says it cannot afford to keep sending empty planes to pick up passengers on many different dates.
Customers due to return from Spain in early August have been told to return earlier.
However, the BBC understands that passengers meant to be flying with the firm on Friday 31 July and Saturday 1 August are not affected – their flights are scheduled as normal.
Several families on holiday on the island of Mallorca, who were due to return next week, have told the BBC they have received emails and text messages telling them they have to return early.
Jet2 did not immediately respond to the BBC’s request for comment.
Some travellers say they are in the dark, because Jet2 – the UK’s largest tour operator – has not confirmed which day they are now meant to return to the UK.
Simon Fordy, from Cumbria, is currently in Magaluf with his family. He is feeling “confused and upset” by the news, having spent almost £3,000 on an 11-day holiday.
Mr Fordy said the family is only five days into their summer break. He told the BBC he is now “constantly checking” his mobile phone, in case he receives word from Jet2 that the family needs to leave in the morning.
It is understood that several families in Magaluf have refused to accept an earlier return flight offered by Jet2.
The UK is advising against all non-essential travel to Spain, including the Balearic and Canary Islands.
It also removed Spain and its islands from the list of countries that are exempt from the 14-day quarantine rule, after Boris Johnson warned that there were signs of a “second wave” of coronavirus in Europe on Tuesday.