Airlines are being investigated by the competition regulator over concerns they failed to offer refunds to passengers when they could not travel.
The Competition and Markets Authority (CMA) will look at cases during England's second lockdown when people were unable lawfully to travel for non-essential purposes.
Flights were still operating during this period.
The CMA will consider whether refusing cash refunds breached consumer rights.
It said that in some cases where flights were not cancelled, customers were told to rebook or offered a voucher rather than a refund.
Andrea Coscelli, chief executive of the CMA, said: "We will be carefully analysing all the evidence to see whether any airlines breached consumers' legal rights by refusing people cash refunds for flights they could not lawfully take.
"We recognise the continued pressure that businesses are currently facing but they have a responsibility to treat consumers fairly and abide by their legal obligations."
The CMA said it would write to "a number of airlines" to request information about their approaches to refunds for passengers prevented from flying by lockdown restrictions.
It will analyse the evidence before deciding whether to launch enforcement action against individual carriers.
The watchdog said it would work closely with aviation regulator, the Civil Aviation Authority ,during its investigation.
Rory Boland, travel editor at consumer association Which?, said: "Airlines have often put customers in an impossible situation by operating flights during lockdown restrictions and refusing to offer cash refunds to people who cannot lawfully travel - so it is right that the CMA has stepped in to investigate and it should take strong action, where appropriate."
He said he expected hundreds of thousands of people who were following government rules by not taking flights to be issued refunds or given the option of a refund, as a result of this investigation.
What are my rights?
If you have a package holiday cancelled by the provider, then a refund should be provided for the whole holiday within 14 days
If your flight is cancelled, you are entitled to a full refund to the original form of payment within seven days, although many airlines are struggling to meet that deadline. You can accept, or refuse, vouchers or a rebooking, but a voucher will probably be invalid if the airline goes bust later
If you decide against going on a future flight, which is not yet cancelled, then there is no right to a refund. Different airlines have different rules over what you can do, but many are waiving any charges for changing to a later flight or having a voucher instead. Your travel insurance is unlikely to cover you