British Airways and Ryanair face a probe over how they handled refunds while travel restrictions were in place.
The UK Competition and Markets Authority is looking into whether the airlines broke the law by failing to give money back for flights that people could not legally take.
During periods of lockdown BA refused to give refunds, instead offering vouchers or rebooking.
Ryanair gave the option to rebook.
Now the CMA says the pair may have breached consumer law.
It has written to both airlines to seek refunds or other redress for affected customers.
In a response, Ryanair said it had handled refunds on a case-by-case basis, and paid money back where justified.
The budget airline said millions of customers had taken advantage of the opportunity to rebook without any fee.