British Airways on Saturday cancelled all flights from London’s Heathrow and Gatwick airports after a global information technology failure. The airline said delays could continue until Sunday, but that they hoped to resume most of them during the day.

The airline has hundreds of flights operating out of the London airports on a normal day, so many thousands of passengers have been affected. On Saturday alone, more than a 1,000 flights were impacted by the outage, The Guardian reported.

British Airways said the problem was because of a “power supply issue” and not a cyber attack. However, Britain’s General Union or GMB blamed the outage on the airline’s decision to outsource several IT jobs to India last year.

Other airlines were not affected. Movement of baggage was frozen after the IT failure, and passenger details were not accessible either. Aviation expert Julian Bray told the BBC, “This is a very serious problem, they should have been able to switch to an alternative system – surely British Airways should be able to do this”.

Customers were asked to check the airline’s website and its Twitter for any updates.