British Airways has started emailing passengers to cancel flights two weeks ahead of another strike by pilots.
The industrial action on 27 September is expected to affect hundreds of flights and tens of thousands of passengers.
The dispute is over a pay-rise which pilots say is not high enough.
Pilots staged a 48-hour walkout earlier this week, forcing nearly 200,000 passengers to change their travel plans.
A spokeswoman for BA said the decision had been made “to give customers as much certainty as possible”. Passengers were being offered a full refund or to rebook on an alternative date or airline.
“We are very sorry that Balpa’s actions will affect thousands more travel plans,” she said. “We urge them to call off their strike and return to negotiations.”
The airline had previously estimated the strike would cost it £40m a day.
Balpa called the decision to cancel flights “irresponsible and inconsiderate to its customers” and said it was a strategy designed to save the airline money.
By giving passengers two weeks’ notice, the airline avoids having to pay compensation.
Balpa said it had set the second strike period at the later date to allow time for negotiations to take place, but that BA had not responded to its latest proposals.
The union said it had given BA “multiple opportunities” to work with them to avoid the strike.
British Airways (BA) has offered pilots a pay rise of 11.5% over three years, which it says would boost the pay of some captains to £200,000.
However, Balpa says its members want a bigger share of the company’s growing profits.
When BA cancelled flights a fortnight ahead of the two-day strike earlier this week, it mistakenly cancelled extra flights as well.
The airline was forced to draft in extra customer relations staff over the August bank holiday weekend to deal with hundreds of thousands of phone calls and messages, after customers found they could not get through to change their flights.