British Airways has cancelled its entire flight schedules for both Heathrow and Gatwick airports Saturday after a global IT failure that fell on one of the busiest days of the year for British holidaymakers.

The carrier, part of the International Consolidate Airlines Group  (ICAGY) , had hoped to clear the system failure, which developed early Saturday morning, and clear passengers from both London airports after 6pm BST. However, the BA was forced to clear the entire schedule and admit that further delays could disrupt travel in the coming days.

"Following the major IT system failure experienced earlier today, with regret we have had to cancel all flights leaving from Heathrow and Gatwick for the rest of today, Saturday, May 27," BA said in a statement published on its website. "We are working hard to get our customers who were due to fly today onto the next available flights over the course of the rest of the weekend. Those unable to fly will be offered a full refund."

"The system outage has also affected our call centres and our website but we will update customers as soon as we are able to," the statement added. 

BA CEO Alex Cruz said the company believes the "root cause" of the system failure was related to power supplies and that there was no evidence of a cyber attack. 

British Airways generated around half of IAG's £22.5 billion ($28.8 billion)  in total revenue last year, according to the company's 2016 annual report. Heathrow Airport moved around 75.7 million passengers last year, its website indicates, and normally handles around 206,000 passengers each day from around 1,200 flights.

Gatwick moved 3.8 million passengers last month, according to its most recent traffic update, an average of around 127,00 per day.

The current holiday weekend, which precedes a week of school holidays for most schoolchildren in Britain, is one of the busiest travel days of the year, according to data from the Civil Aviation Authority.