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

A glitch in American Airlines' online holiday booking system left the carrier trying to find flight crews for a total of 15,000 departures over the busiest period of the year. AA pilots who had booked time off between December 17-31 are currently being offered more money if they cancel holiday plans and return to work.

APA (Allied Pilots Association), which represents over 10,000 American Airlines pilots, revealed the carrier was facing a “crisis”.

After AA pilots found out about the error, many have been offered time-and-a-half to work over Christmas.

American Airlines is the planet's largest airline by most measures, including total passengers (198,700,000 in 2016), fleet size (1,556) and annual revenue (US$40.99 billion). It operates services to 4 UK cities: Edinburgh, Manchester, Glasgow and London.