LONDON: British Airways (BA) cabin crew launched a four-day strike Saturday, the second wave of action in a week as part of a bitter, long-running dispute over pay and conditions.

BA has pledged that more than three-quarters of its passengers -- or over 180,000 out of 240,000 -- will still be able to travel as planned during the walkout, which follows a similar three-day action last week.

Another 18 percent of its customers have been rebooked with other airlines, or have switched their travel dates to avoid the strike period, it said.

BA chief executive Willie Walsh said the "vast majority" of its staff were "pulling together to serve our customers and keep our flag flying".

"At the same time, I feel really sorry for those customers whose plans have been ruined by the Unite union's completely unjustified action," he added.

Amid growing hostility between BA and trade union Unite, whose members are staging seven days in total of strikes, the union claimed the cost to BA would be 100 million pounds (111 million euros, 149 million dollars).

"If you add together the cost of lost bookings, of revenue effectively transferred to other airlines along with BA passengers, the cost of (leased) aircraft and the cost of knock-on post-strike disruption, this is the ball-park area we are in," the union said in a statement.

By contrast, BA said Monday that a three-day walkout from last Saturday would cost seven million pounds a day and that an assessment of the full cost of the seven-day action could only be made after it was finished.

Talks between the two sides broke down eight days ago, on the eve of the first wave of strikes.

Walsh came under fire from leading academics in a letter to The Guardian newspaper published Friday.

The letter, signed by 116 industrial relations experts from universities across Britain, argued that Walsh's actions were designed to break Unite, which represents BA's 12,000 cabin crew.

They added that Walsh had withdrawn an offer which could have prevented Unite's strikes -- and used airplanes from fierce rival Ryanair to combat last weekend's strike.

BA has also axed highly-prized perks like big travel discounts for all striking workers.

"It is clear to us that the actions of the chief executive of British Airways... are explicable only by the desire to break the union which represents the cabin crew," the academics' letter read.

"What other possible interpretation can there be for Willie Walsh tabling an inferior offer to BA's previous one, or indeed his marshalling of resources, including those of bitter rival Ryanair, to undermine the action of his staff?"

BA issued a curt response to deny that it was seeking to break Unite.

"We reject the suggestion that BA's real objective is to break Unite," the carrier said in comments emailed to journalists.

BA is hoping to fly a full and normal schedule from London's City and Gatwick airports over the next four days.

At London Heathrow, one of the world's busiest airports, the carrier said it would operate 70 percent of long-haul and 55 percent of its short-haul flights.

The airline, which is attempting to merge with Spanish rival Iberia, said last month it expected to notch up a record loss in the current financial year due to weak demand for air travel.

In December, BA won a legal battle to prevent a 12-day walkout by cabin crew over Christmas and New Year after a judge ruled that a ballot of staff by Unite was invalid.

Prime Minister Gordon Brown has condemned the strike.

But with a general election expected to take place in May, the main opposition Conservatives have accused the government of a weak response to the strike because Unite is a major donor to Brown's ruling Labour party.