British Airways cancelled all its flights from Heathrow and Gatwick airports in London on Saturday after a 'major IT system failure', leaving thousands of travellers stranded as a workers' union blamed outsourcing to India for the 'meltdown'.

The airline initially cancelled only flights before 6 pm local time on Saturday but later confirmed all its flights will remain grounded at both airports throughout the day.

"Our IT teams are working tirelessly to fix the problems," British Airways CEO Alex Cruz said in a video posted on Twitter.

"We believe the root cause was a power supply issue and we have no evidence of any cyberattack."

The GMB union said the 'meltdown' could have been avoided if BA hadn't made hundreds of IT staff redundant and outsourced their jobs to India at the end of last year.

"This could have all been avoided. In 2016 BA made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India," said Mick Rix, national officer for aviation at GMB.

"BA have made substantial profits for a number of years,and many viewed the company's actions as just plain greedy," he said. -- PTI