British Airways cancelled nearly 60% of its flights on May 27 when an IT outage knocked out the airline's systems and stranded 75,000 people over a holiday weekend.
The airline cancelled 479 flights, or 59% of its operations on May 27, and 193 flights (23%) of its operations on May 28, its parent International Consolidated Airlines Group said.
It blamed a power surge that knocked out its computer system, disrupting flight operations, call centres and its website.
The flights resumed on May 30 but the company has faced increasing pressure over its response to the IT failure.
IAG chief executive Willie Walsh said on Monday that British Airways had commissioned an independent study into the shutdown.
"British Airways is working hard to compensate affected passengers as quickly as possible," said IAG, which also includes Aer Lingus and Spanish airlines Vueling and Iberia.
British prime minister Theresa May had called on British Airways last week to compensate the thousands of passengers who were left stranded.
British Airways' traffic, measured in revenue passenger kilometres, fell 1.8% in May, while it rose by 1.8% for the group as a whole.
The group's capacity, measured in available seat kilometres, rose by 0.9% in May, while for British Airways it fell 1.9%.