The IT meltdown at British Airways forced the airline to cancel more than 670 flights over two days.
The power failure over the late May bank holiday weekend led to severe disruption of flights.
Parent company International Airlines Group revealed that BA cancelled 479 flights representing 59% of operations on May 27.
A further 193 flights, or 23% of operations, were grounded the following day.
The airline was flying the vast majority of its schedule by Monday, May 29, but it now faces compensation claims from thousands of affected passengers, many who had school half term holidays ruined.
IAG said: “An independent investigation will examine every aspect of the power failure.
“British Airways is working hard to compensate affected passengers as quickly as possible.”
The level of flight cancellations emerged as IAG issued statistics for May which saw group traffic edge up by 1.8% over the same month last year.
Premium traffic across the group, which also includes Aer Lingus, Iberia and Vueling, rose by 5.2% year-on-year.
Sales for IAG’s new low cost, long haul offshoot Level are ahead of expectations with more than 134,000 tickets sold since becoming available on March 17.
Level’s first Airbus A330 flight left with all 314 seats full between Barcelona and Los Angeles on June 1, followed by the start of three flights a week to Oakland the next day.
“This was Spain’s first long haul, low cost operation and is the first step towards Level’s expansion from other European cities,” IAG said.
“In 2018, Level will increase its fleet to five aircraft. Level’s sales are well ahead of expectations in all markets.”