The Competition and Markets Authority (CMA) received thousands of complaints from people not receiving refunds for cancelled Tui holidays within 14 days.
Consumer protection law requires refunds within 14 days.
The vast majority of people have already received their refunds or rebooked during the CMA’s investigation.
Any outstanding refund requests for people who had their package holiday cancelled as a result of coronavirus will be paid by September 30.
Some customers may have accepted refund credit notes. The CMA said it was important these customers know they are entitled to a cash refund as an alternative.
Tui has also agreed to contact customers who have an unused credit, to let them know they can convert it to a refund, which will be paid within 14 days.
Andrea Coscelli, the CMA’s chief executive, said: “It’s absolutely essential that people have trust and confidence when booking package holidays and know that if a cancellation is necessary as a result of coronavirus, businesses will give them a full, prompt refund.
“The CMA’s action ensures that Tui customers will get their refunds by the end of the month.
“The CMA is continuing to investigate package holiday firms in the context of the Covid-19 crisis.
“If we find that businesses are not complying with consumer protection law, we will not hesitate to take further action.”
The CMA wants to ensure that Tui meets its obligation to pay refunds on time in the future. Therefore, Tui will report regularly to the CMA over the coming year on the time it has taken to refund customers.
These commitments apply to all of Tui UK’s different businesses that offer package holidays, including First Choice, First Choice Holidays, Marella Cruises, Crystal Ski, Crystal, Tui Scene, Tui Lakes & Mountains and Skytours.
Today’s announcement follows significant action by the CMA on cancellations in the holiday sector.
It has written to over 100 package holiday businesses to remind them of their obligations to comply with consumer protection law and has opened investigations into a number of operators.