The systems failure that grounded thousands of British Airways passengers will cost the airline at least £80 million.

Willie Walsh, chief executive of BA owner International Airlines Group, told investors its initial assessment of the gross cost “is in the order of £80 million”.

The airline was forced to cancel more than 670 flights over two days over the late May bank holiday weekend.

Walsh promised to provide an update with more details when appropriate, the BBC reported.

The chaos was reportedly triggered by an engineer who disconnected a power supply.

The major damage was caused by a surge when it was reconnected.

Speaking at IAG’s annual meeting, Walsh apologised again for last month’s chaos, describing it as a “dreadful experience”.

About 75,000 passengers faced severe disruption when BA’s system failed.

Mr Walsh said BA was “working hard to ensure that affected passengers are compensated as soon as possible”.

He had previously announced an independent investigation “to learn from the experience”.

Walsh said: “What we do know at this stage, however, is that this failure had absolutely nothing to do with changes to the way we resource our IT systems and services.”

IAG chairman Antonio Vazquez said: “You can be reassured that we will do everything in our powers to avoid similar problems in the future and restore our customers’ faith in British Airways.”

BA was one of several airlines affected by a baggage system problem at Heathrow this morning.

Hundreds of passengers on flights operated by IAG-owned Iberia, as well as BA and American Airlines at Terminals 3 and 5 had to fly out without their luggage.

Heathrow said the problem has now been fixed and apologised.