Teletext Holidays is being investigated after hundreds of customers complained they were denied refunds for cancelled trips.
The Competition and Markets Authority (CMA) said it has launched an investigation under consumer protection law.
Some complainants reported they were promised refunds for package holidays by a certain date, which was later pushed back.
People who book a package holiday are protected under the Package Travel Regulations, which mean they should get their money back within 14 days depending on the reason for the cancellation.
CMA chief executive Andrea Coscelli said: “We understand that the pandemic is presenting challenges for travel businesses, but it is important that the interests of consumers are properly protected and that businesses comply with the law.
“We’ll be engaging with Teletext to establish whether the law has been broken and will take further action if necessary.”
The CMA has carried out inquiries into the actions of a number of travel firms during the pandemic.
In December, it launched a probe into airlines following concerns they breached consumer rights by failing to offer cash refunds for flights passengers could not take.
It had previously secured refund commitments from Lastminute.com, Virgin Holidays, Tui, LoveHolidays, Sykes Cottages and Vacation Rentals.