British Airways is cancelling almost all services on 9-10 September owing to the unpredictability caused by a pilot strike.

The airline, which carried nearly 4.6 million passengers during August, says it has "no option" but to cancel "nearly 100%" of its flights as a result of the industrial action called by cockpit union BALPA.

Oneworld alliance member BA says the union has given "no detail" on which crews would participate in the strike.

"We had no way of predicting how many would come to work or which aircraft they are qualified to fly," the carrier adds.

It says it is working with partner airlines to recruit larger aircraft to maximise capacity on affected routes.

The strike is particularly frustrating for BA which has been engaged in a public campaign to celebrate its 100th anniversary during 2019.

"After many months of trying to resolve the pay dispute, we are extremely sorry that it has come to this," it says. "We remain ready and willing to return to talks with BALPA."

The airline has been discussing pay proposals with its main unions since last November, and it points out that the two largest unions – Unite and GMB – have recommended accepting the "generous" offer of an 11.5% increase over three years.

But this offer has been rejected by BALPA. The union says it offered to call off the 9-10 September strike if BA had been prepared to discuss a new proposal put forward by the union.

It accuses BA of being prepared to "inflict this bullying tactic on its staff and see its passengers take the brunt of the strike" than engage with its employees.

BALPA says the airline's pilots "stand united" in seeking a better deal, adding that they "remain very angry" with the carrier. Another strike is scheduled for 27 September.

The airline, the primary operator of parent company IAG, made a full-year operating profit of £1.95 billion ($2.4 billion) during 2018 on revenues of just over £13 billion.

BALPA claims the strike will cost BA some £40 million per day but that the union's latest proposal amounts to less than £5 million more than a figure previously offered by the airline to settle the dispute.

The union says the airline's response is "illogical and irresponsible" and will be detrimental to its relationship with cockpit crews.

BA's CityFlyer services and its Sun-Air and Comair franchise operations are not affected.