British Airways pilots are to strike on 9, 10 and 27 September in a dispute over pay, the British Airline Pilots Association (Balpa) has said.
The three-day strike - the first by BA pilots - could cause severe disruption, as Balpa represents about 90% of the airline's pilots.
The union said strikes were a "last resort" born out of "enormous frustration" with airline management.
BA called it "unjustifiable" and said Balpa was "destroying" travel plans.
The union said BA rejected a "number of packages" that were proposed to resolve the pay dispute, and that there remains a gap of around £5m between the two sides' positions.
The industrial action comes after 93% of Balpa's members voted in favour of striking.
"Over recent years BA pilots have made sacrifice after sacrifice to assist the company such as taking a pay cut, productivity increases, closing the final salary pension scheme, giving up annual leave days, a new rostering system, and reducing flying pay," Balpa said.
Pilots have rejected a pay increase worth 11.5% over three years, which the airline put forward in July.
BA's chairman and CEO, Alex Cruz, said it was a "fair deal" and called the move by Balpa "disappointing".
BA said it is making changes to its flight schedule, adding that it will "do everything [it] can to get as many people away on their journeys as possible".
"However, it is likely that many of our customers will not be able to travel and we will be offering refunds and rebookings for passengers booked on cancelled flights," it said.
Speaking to BBC News, Mr Cruz said it remains unclear how much disruption the strikes may cause.
"We are focused on minimising disruption to those who may be affected," he said.
BA lost an appeal aimed at halting the strikes last month.
Its owner, International Airlines Group (IAG), had sought an injunction to prevent the strike in the High Court, but it was overturned.
It was then rejected by the Court of Appeal on 31 July.
IAG, which also owns Spanish carrier Iberia and Ireland's Aer Lingus, reported a pre-tax profit of €3bn (£2.7bn) last year, up almost 9.8% on the previous year.
British Airways contributed £1.96bn to that, up 8.7% on 2017.