LONDON - British Airways is facing the threat of more strikes over the summer after union leaders said they would ballot cabin crew members for further action in a bitter dispute over changes to pay and working conditions.

As the last of 22 days of walkouts - which since mid-March have cost the airline more than 150 million pounds ($220 million) - drew to a close on Wednesday, there was little sign of a resolution to the dispute, with BA boss Willie Walsh vowing to "hold out as long as it takes."

The airline continued to operate a 6:10 p.m. nonstop flight from Philadelphia International Airport to London, but canceled a 9:15 p.m. daily flight from here during the strike. British Airways spokesman John Lampl said that the second daily flight would resume on Friday.

Though BA has been forced to cancel hundreds of flights because of the walkouts in March, May and June, it has steadily increased services during the strike period as fewer workers participated in the action and it used quickly-trained staff from other departments.

The airline has also used leased planes and crew and booked its passengers onto flights with rival airlines in a bid to minimize the disruption, allowing it to operate around 80 percent of its long-haul flights from Heathrow in the last five-day walkout.

Services from Gatwick and London City haven't been affected at all - a far cry from the last major strike in 1997, when the airline was forced to shut down completely.

The airline's relative success at keeping a majority of booked passengers flying has seen the dispute fade from the front pages of newspapers and the lead item on TV bulletins.

Walsh, who has been backed by BA investors, told industry leaders at a conference in Berlin that he was "absolutely determined" not to give in.

"We're going to hold out for as long as it takes and we will continue to build up the amount of flying we're doing," he said.

"I don't think we've been brave enough in the past to stand up and say, 'No,' " he added.

The combative Irish CEO has repeatedly warned that contentious changes to work practices, including fewer staff on long-haul flights and a yearlong pay freeze, are necessary for BA to survive.

The airline has been hit hard by the financial downturn because of its heavy reliance on premium fare passengers on the trans-Atlantic route, posting a record annual loss of 425 million pounds ($611 million) last month. As many leisure and business travelers seek lower cost options, there are fears that its core business will never fully recover.

The government-backed conciliation service Acas has said it expects a date to be set shortly for peace talks to resume, but while the two sides are close to agreement on many issues, mistrust and bitterness have made a deal elusive.

The major sticking point is now BA's refusal to reverse its decision to cut travel perks for striking workers and take disciplinary action against some staff.

Unite assistant general secretary Len McCluskey told a noisy rally of strikers near Heathrow Airport that the "fight for justice" would continue, adding that preparations for a fresh ballot were at an advanced stage.

The union has promised not to take any action during the football World Cup in South Africa, and mid-July would be the likely timing for a new wave of stoppages.

McCluskey said the blame for the continuing instability at the airline must be placed "firmly" at the door of BA management's "tough guy" stance.

"Comments about holding out for as long as it takes against the workforce should cause despair among BA board and shareholders," he said. "This bunker mentality suggests that the true objective is not cutting costs, but crushing the workforce. Talking tough will not help find peace."

Another ballot will test the appetite of workers for more strike action amid suggestions that many are losing heart for a long-fought campaign.

BA, meanwhile, is trying to shore up passenger support, announcing a premium fare sale this week for travel in the second half of the year.

Announcing the deals, Walsh acknowledged that passengers "may be concerned about the possibility of future disruption but it is our intention to keep flying 100 percent of our long-haul operation throughout the coming months."