The British Airways strike next month is to cause at least five consecutive days of flight cancellations, the airlines said on Saturday.
British Airways pilots announced three days of strikes - taking place on September 9, 10 and 27 - in a dispute over wages, the BBC reported.
The British Airline Pilots Association (Balpa) said on Friday the strikes were a "last resort" born out of "enormous frustration" with airline management.
The pilots have rejected a pay increase worth 11.5 per cent over three years, which the airline put forward in July.
Despite the first strikes being on September 9 and 10, some customers flying between the 8th and 12th have been told their flight has been cancelled and to rebook or get a refund.
"We are doing absolutely everything we can to prevent this unfair action from taking place and ruining our customers' travel plans.
"Airlines have a very complex operation and during times of widespread disruption, there can be knock-on effects onto flights on other days," the UK flag carrier said in a statement.
Customers have reported receiving emails late Friday night and in the early hours of Saturday morning informing them their flight had been cancelled.
Many have taken to social media to complain that they have been unable to rebook via the website or get through on the airlines' phone lines.
According to the airline, it carries 145,000 customers every day, with a fleet of more than 280 aircraft.