The world’s largest airline could be forced to ground thousands of flights over the festive period after giving too many of its pilots time off work.

A glitch in the online holiday booking system used by American Airlines has left the carrier scrambling to find flight crews for 15,000 departures over one of the busiest periods of the year. Pilots who had booked time off from December 17-31 are now being offered more money to cancel their holiday plans and return to work.

The Allied Pilots Association (APA), which represents more than 10,000 American Airlines pilots, said the carrier was facing a “crisis”.

“The [holiday booking] system went from responsibly scheduling everybody to becoming Santa Claus to everyone,” said Dennis Tajer, a spokesman for the union. “The computer said, ‘Hey ya'll. You want the days off? You got it.’ I'm watching a Grinch that stole Christmas thing happening. And we don't want to see that happening for our passengers.”

Pilots found out about the error on Friday and many have since been offered time-and-a-half to work over the Christmas period.

Pilots have since been offered time-and-a-half to work over the Christmas period Credit: GETTY

“We are working diligently to address the issue and expect to avoid cancellations this holiday season,” said an American Airlines spokesman. “We have reserve pilots to help cover flying in December, and we are paying pilots who pick up certain open trips 150 percent of their hourly rate - as much as we are allowed to pay them per the contract. We will work with the APA to take care of our pilots and ensure we get our customers to where they need to go over the holidays.”

The crisis is reminiscent of Ryanair’s cock-up earlier this year, when a rostering issue forced the airline to cancel tens of thousands of flights. In excess of 700,000 passengers were affected by the fiasco, costing the Irish carrier millions in lost revenue.

American Airlines is the largest airline on the planet by most measures, including annual revenue ($40.99bn), total passengers (198,700,000 in 2016) and fleet size (1,556). It operates services to four UK cities: London, Glasgow, Edinburgh and Manchester.

Should UK passengers have their flight cancelled they should expect a refund and cash compensation under EU legislation that applies to all services flying to or from an EU airport.