British Airways and Ryanair are being investigated over flights being cancelled due the pandemic.
It's to be decided whether they broke competition laws by not offering refunds to travellers.
The Competition and Markets Authority (CMA) on Wednesday opened enforcement cases into both firms and has written to them with concerns.
Ryanair currently flies from both Belfast International Airport and Belfast City Airport, with BA flying from just the city.
The CMA previously opened an investigation into airlines in December 2020 following reports that consumers were being denied refunds for flights that they could not legally take due to Covid-19.
During different lockdowns here, both British Airways and Ryanair refused to give refunds to customers who were lawfully unable to fly.
British Airways offered vouchers or rebooking and Ryanair also proved the option to rebook.
The CMA is concerned that, by failing to offer people their money back, both firms may have breached consumer law and left people unfairly out of pocket. It is now seeking to resolve these concerns with the companies, which may include seeking refunds, or other redress, for affected customers.
Andrea Coscelli, Chief Executive of the CMA, said: "While we understand that airlines have had a tough time during the pandemic, people should not be left unfairly out of pocket for following the law.
"Customers booked these flights in good faith and were legally unable to take them due to circumstances entirely outside of their control. We believe these people should have been offered their money back."
These airlines now have the opportunity to respond to the CMA.
For more information, visit the COVID-19: lockdown flight refunds page.
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