The system meltdown that left 75,000 British Airways passengers stranded at the start of the May half-term holiday cost the airline £80m, shareholders have been told.

At the annual meeting in Madrid of BA’s parent company, IAG,Willie Walsh gave the airline’s first assessment of the fiasco’s gross cost, including rebooking and compensating passengers and restoring lost luggage.

Walsh, the IAG chief executive, said the group would continue to update shareholders on the bill. He said the power failure led to BA cancelling 726 flights , 28% of scheduled flights over three days.

Walsh apologised again for the incident, adding: “I know it was a dreadful experience for many of our customers.”

But he continued to praise BA bosses and rejected any link to decisions to outsource IT and other jobs.

He said that while they awaited the results of an independent investigation into the power failure at BA’s primary centre on 27 May, “what we do know is that the failure had absolutely nothing to do with the way we resourced IT”. Walsh credited the contractors and BA staff with an “outstanding achievement” to restore services, adding: “Despite major operational problems, by 29 May it was able to run the majority of its schedule.”

IAG has already attributed the cause of the crisis to an error by a technician who cut the power and restored it “in an uncontrolled manner”.

Walsh’s praise was matched by the IAG chairman, Antonio Vázquez Romero, who said: “It is only right to recognise the efforts of the BA team and the efficiency with which they restored operations.”

He promised they would “do everything to restore customers’ faith”.

Romero said the last year had seen “very difficult circumstances” for the airline group, with the Brexit vote and subsequent collapse of the pound hitting profits. IAG is heavily dependent on BA’s sterling revenues, but major costs such as fuel and aircraft are paid in dollars, while the group reports in euros.

Despite the currency fall having a €460m (£402m) adverse impact, IAG made €2.53bn in pre-tax profits and Romero said: “Our long-term financial goals remain practically unchanged [by Brexit]”. He added: “Our commitment to adequately remunerating our shareholders is unwavering.” Almost €500m in dividends was paid out last year.

Walsh said IAG would continue to expand in low-cost, long-haul flying after “an incredible customer response” to the launch of its new airline brand Level in Spain last month, with 52,000 tickets sold in the first 12 hours of sale.

Walsh also took the opportunity to attack the proposals for a third runway at Heathrow. He said it was “a great opportunity for IAG – but the cost of new infrastructure must not be paid for by consumers”.

He added that the airport’s £17.6bn price tag was “unacceptable... and would definitely mean hiked prices for consumers”.