Wizz Air has had to pay out an extra £1.2m to customers, after the industry regulator intervened over the way it handled flight disruption.
The Civil Aviation Authority (CAA) got involved in July after it received "high volumes" of complaints about the airline not paying what it owed.
The CAA told Wizz Air to review thousands of rejected claims for costs.
Wizz Air said it had fully co-operated with the regulator and put improvements in place.
Paul Smith, consumer director at the CAA, said the outcome was good news for passengers, but that such interventions should not be necessary.
"While we welcome the steps taken by Wizz Air after falling short in its treatment of disrupted passengers, airlines should routinely look after passengers and uphold their rights when flights are delayed and cancelled," he said.
Wizz Air looked again at 25,000 claims for flights to or from the UK from 18 March 2022 onwards. Of those, 6,000 have resulted in additional payments, with the amounts received varying significantly.
Not all of the claims had been initially rejected but of the 6,000 some will have previously received nothing. Others who received partial refunds will have seen these topped up.
Wizz Air, which is based in Hungary, operates low-cost short haul flights in Europe.
The airline had faced "unprecedented operating challenges" in summer of 2022 said Wizz Air UK managing director Marion Geoffroy.
One of those who suffered in that disruption was Russell Quirk, a property expert from Brentwood, in Essex. He found it very difficult to claim back the money owed to him after his family's flights were cancelled at the last minute.
After months of waiting for reimbursement he finally resorted to sending bailiffs to Luton Airport to confront Wizz Air.
Wizz Air paid up at the time, apologised and said it "fell short of our own aspirations and our customers' expectations".
Since then the airline has invested £90m in improving operations and launched new initiatives to reduce cancellations and improve performance, including adding spare aircraft capacity and increasing staffing at airports and claim centres.
- Flying from a UK airport
- Arriving at a UK airport on an EU or UK airline
- Arriving at an EU airport on a UK airline
Airlines must offer the choice of a refund or alternative flight, regardless of how far in advance the cancellation was made.
Return tickets must be fully refunded if either leg is cancelled and passengers have a right to be booked on another airline - or other suitable mode of transport - if it is going to their destination significantly sooner.
The consumer group Which? had suggested that the CAA should take the airline to court if it did not fulfil legal requirements.
"Wizz Air has an abysmal record on meeting its legal obligations under consumer law, racking up millions of pounds in county court judgements after continually failing to appropriately reroute passengers," its travel editor, Rory Boland, claimed.
Wizz Air has been the worst major airline for flight delays from UK airports for two years in a row, according to analysis of CAA data, although the company said it had improved punctuality in 2023.