The U.K.’s Competition and Markets Authority is investigating whether airlines violated consumers’ legal rights by failing to offer cash refunds for flights they couldn’t lawfully take.

The probe is part of an ongoing review of holiday refunds during the coronavirus pandemic, the CMA said Wednesday in a website statement. Some airlines kept operating flights even if lockdown rules in the U.K. or abroad made it impossible for passengers to fly legally, it said.

The refund issue has been a contentious one since the pandemic disrupted travel earlier this year, as airlines starved for cash steered passengers toward credits for future flights. The failure to offer prompt refunds spurred a backlash.

During the second lockdown in November, when rules prevented most non-essential travel, some passengers were offered the option to rebook or to receive a voucher instead of cash back, according to the CMA.

“We recognize the continued pressure that businesses are currently facing, but they have a responsibility to treat consumers fairly and abide by their legal obligations,” CMA Chief Executive Officer Andrea Coscelli said in the statement.

The CMA will work closely with the U.K. Civil Aviation Authority in the investigation, it said.