British Airways has been forced to cancel all passenger flights out of Heathrow for two days next week because of a strike by thousands of cabin crew.
Domestic and European flights to and from Gatwick will also be grounded on the two days - Tuesday and Wednesday - following the breakdown of talks aimed at resolving a bitter row over sickness absence, pay and staffing.
Hundreds of thousands of passengers will be hit by the news, which will lead to hundreds of flights being cancelled.
BA chief executive Willie Walsh said: "We are deeply sorry that our customers are the innocent victims of this unnecessary and unjustified strike."
The airline said it was still trying to reach a deal with the Transport and General Workers Union before the start of the strike next Tuesday but wanted to give customers early warning of its schedule to give them time to make alternative arrangements.
The union has called two further three-day walkouts from February 5 and February 12 unless the deadlock is broken.
Mr Walsh said: "More than 15,000 customers a day have contacted us since the union announced a series of 72-hour strikes, extremely concerned about their winter holidays and business trips.
"Announcing our contingency plans means we can end uncertainty for customers caught up in the first round of strikes and help them make other plans.
"If we postponed the cancellation of flights until the eve of a strike, customers would have virtually no time to make alternative arrangements.
"We remain absolutely determined to search for a negotiated settlement and our door remains open to the T&G, day or night. We regret that the T&G has not supported our initiative to seek the assistance of the Acas conciliation service.
"It is not too late for the T&G to call off this dispute and we will do all we can to reinstate some of the cancelled flights."
The airline said it will allow any customer due to travel on a British Airways flight cancelled by the strike to claim a full refund, rebook their flight for a later date or be rebooked by BA with another airline.
BA said customers due to travel on a cancelled service should not go to Heathrow or Gatwick.
The airline said its flight programme was complex, involving a combination of rosters for 15,000 cabin crew, 3,000 pilots and 234 aircraft operating up to 750 services in and out of the two airports.
More than 8,000 crew have to be in the right place at the right time, either on aircraft, at airports or in hotels in more than 140 cities in 75 countries, every day.
BA also warned of further cancellations on either side of the official strike dates because crew and aircraft will be out of position.
An extra call centre has been opened by BA, staffed by 100 workers, to help with customer inquiries.
Long-haul aircraft will leave Heathrow on Tuesday and Wednesday without passengers, to pick up crew and allow inbound flights to operate.
Up to six of the nine daily long-haul flights from Gatwick will operate normally.
Customers wishing to rebook their flights can contact the airline on 0800 727 800.
Meanwhile, the GMB union wrote to BA seeking an urgent meeting in an attempt to resolve a separate row over pensions.
The union is recommending rejection of a plan to tackle the airline's £2.1 billion pensions deficit.
National officer Ed Blissett said: "We are deeply concerned that BA management has misrepresented the GMB's position with regard to progress of the pension talks alleging that GMB would recommend the latest offer to the members.
"This was never the GMB's position and BA knew that all along. There is now a lack of trust between BA management and GMB."