Members of the British Airline Pilots Association (Balpa) are taking their first ever industrial action against the airline, grounding hundreds of flights.
BA has offered a pay rise of 11.5% over three years but Balpa says its members wanted a bigger share of the company's profits.
BA said in a statement: "We understand the frustration and disruption Balpa's strike action has caused our customers. After many months of trying to resolve the pay dispute, we are extremely sorry that it has come to this.
"We remain ready and willing to return to talks with Balpa.
"Unfortunately, with no detail from Balpa on which pilots would strike, we had no way of predicting how many would come to work or which aircraft they are qualified to fly, so we had no option but to cancel nearly 100% of our flights."