The measure affects passengers who purchased non-refundable tickets for Ryanair flights that operated as scheduled during the pandemic but were unable or unwilling to fly because of travel restrictions imposed by governments. Ryanair (RYAAY) said in a statement on Tuesday that fewer than 1,000 people are affected, adding that they "chose not to travel and then unlawfully processed chargebacks via their credit card company."

They will have to "settle their outstanding debt" with the airline before they will be allowed to fly again, it added. Customers who received refunds directly from the airline after it canceled flights will not be affected, Ryanair said.

Ryanair's attempt to recover the money was first reported by MoneySavingExpert, a personal finance website in the United Kingdom.

Chargebacks occur when a customer disputes a payment on their debit or credit card and has the transaction reversed by their bank, which recoups the money from the merchant's bank.

During coronavirus lockdowns, Ryanair and IAG's (ICAGY) British Airways refused to refund some passengers for flights they could not legally take due to government rules banning non-essential travel. The airlines offered vouchers or the option to rebook instead.

MoneySavingExpert reported that it had spoken to three passengers who said that they discovered only after booking new trips with Ryanair that they would not be allowed to board the plane unless they repaid refunds totaling hundreds of pounds. According to the report, Ryanair offered to refund the new tickets if customers did not wish to return the chargebacks.

But one of the passengers reportedly found out just three days before she was due to travel and felt she had no choice but to repay Ryanair or risk forfeiting money spent on accommodation, car hire, Covid-19 tests and airport parking.

All three passengers had claimed refunds from American Express (AXP) after Ryanair refused to process refunds for the flights they did not take in 2020, MoneySavingExpert said. They told the outlet that they were not offered vouchers or the option to rebook.

In a statement shared with CNN Business, American Express said it reviews all refund claims "carefully and fairly, and on a case by case basis with the merchant and cardmember."

Visa (V) and Mastercard (MA) declined to comment.

According to MoneySavingExpert, dozens of would-be holidaymakers are complaining of a similar issue with Ryanair on the website's forum.

In terms and conditions published on its website, Ryanair says it may refuse to carry a passenger if they owe the airline money in respect of a previous flight "owing to payment having been dishonored, denied or recharged against us." Passengers agree to these rules when booking their tickets.

"I do not see any obvious challenge to this approach," Colin Murphy, a lawyer in the travel team at UK firm Leigh Day told CNN Business. "Airlines have a wide discretion to deny boarding."

Last week, Britain's antitrust regulator dropped an investigation into whether Ryanair and British Airways had broken the law by refusing to refund customers for flights they weren't able to take during lockdowns.

It concluded that the law does not provide passengers who were prevented from flying by travel restrictions with a sufficiently clear right to a refund.

By Hanna Ziady, CNN Business