British Airways has warned that the unprecedented scale of the walkouts over a 23-day period would cause "extensive disruption for potentially hundreds of thousands of customers over a busy period" and may affect travelers to South Africa for the month-long football World Cup, which kicks off on June 11.

Unite, which represents around 90% of BA's 12,000 cabin crew, said it was forced to ramp up its actions in a long-running and increasingly bitter dispute over pay and conditions after BA turned down an approach by the union over the weekend.

"Regardless of the reasons behind these strikes, the people who are going to suffer the most are BA's customers," said Bob Atkinson, of travel website travelsupermarket.com.

"Thousands of families will have booked their trips well in advance and will have little flexibility, minimal realistic alternatives and will be left disappointed."

Unite said it approached BA after its members had turned down the latest offer from the airline on Friday - following advice from angry union leaders from BA failed to restore travel perks taken away from staff who joined in strike action in March.

Union leaders are also unhappy that BA is taking disciplinary action against more than 50 union members and fired an official who represented cabin crew.

In a statement, Unite's joint general secretaries Derek Simpson and Tony Woodley said: "Cabin crew is left with no choice but to take further strike action," adding that "There can be no industrial peace without meaningful negotiations and while management victimizes trade unionists and uses disciplinary procedures in a witch-hunt."

7-day period

The pair said that the required seven days' notice period gave BA time to "reopen meaningful negotiations."

BA, which called its offer "very fair", said it had already begun speaking to a number of carriers about leasing in extra aircraft to support its short-haul schedule and also obtaining thousands of seats from other airlines.

In a positive note for some travelers, BA said that it planned to operate all flights to and from Gatwick as normal.

At Heathrow, it plans to operate "a substantial part" of its long-haul schedule and "a number" of daily flights to every destination across its short-haul network.

The airline said it would give more details four to five days before the strike is due to begin.

"This decision has no semblance of justification," it said in a statement. "We are confident that many crews will again ignore Unites pointless strike call and support the efforts of the rest of the airline to keep our customers flying."

BA has consistently maintained that the series of disputed changes to pay and conditions, including reducing the number of crew on long-haul flights and a pay freeze, are necessary to cut costs after two record years of losses at the airline as the global financial crisis hit passenger demand.

BA estimated that the two strikes by cabin crew in March over seven days cost it around £45m in cancelled flights and contingency provisions, including hiring other aircraft complete with crew.

The airline lost a further £180m last month when the Icelandic volcanic ash cloud grounded flights.