The sale will last for 12 days. It includes flights to 26 destinations, to be taken between 12 September and 16 December, at fares from pounds 16.99.
Michael O'Leary, chief executive of Ryanair, criticised the prices and availability of tickets during BA's 48-hour sale, and said: "Ryanair will have to show BA how a low-fare seat sale should be run."
A spokeswoman for BA said it was unconcerned by the Ryanair offer and had no plans to offer another sale at the moment. "Ryanair's sale demonstrates what we have always said, that competition benefits the consumer, but our sale was much larger and involved more long-haul flights," she said.
"Competition is not restricted to fares alone, it includes service in the air and on the ground, and we offer a full service including food and drink, so the sales do not really compare."
The pounds 16.99 fares will be available on flights from Stansted to Rimini in Italy, Kristianstad in Sweden and St Etienne in France, as well as from Luton and Liverpool to Dublin, and Stansted to Prestwick in Scotland. Flights to Pisa, Stockholm, Carcassonne and Oslo will cost pounds 29.99 one way, but only for weekday travel.
Anyone travelling on Friday, Saturday or Sunday will have to pay pounds 10 extra each way.Reuse content