British Airways has outsourced IT jobs to India, leading to allegations of greed by unions.
The GMB said the disruption could have been avoided if the airline had not cut "hundreds of dedicated and loyal" IT staff and contracted the work to India in 2016.
Mick Rix, national officer for aviation at the union, said: "We can only feel genuinely sorry for the tens of thousands of passengers who are stranded at airports and face having their travel plans and holidays ruined.
"This could have all been avoided. In 2016 BA made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India.
"BA have made substantial profits for a number of years, and many viewed the company's actions as just plain greedy."
The move in 2016 sparked protests and outrage from members of the union.
At the time, a BA spokesman said: "A contract has been signed with TCS to be the supplier of some IT activities in British Airways, and British Airways has been in consultation with those IT staff affected, about 200.
"British Airways employs around 35,000 people in the UK, providing high-skilled and well-paid jobs. It hires 1,000 people a year and has a strong apprenticeship programme."
A BA spokesman said: "We would never compromise the integrity and security of our IT systems.
"IT services are now provided globally by a range of suppliers and this is very common practice across all industries and the UK Government.
"British Airways employs around 35,000 people in the UK, providing high-skilled and well-paid jobs. It hires 1,000 people a year and has a strong apprenticeship programme."