British Airways Plc cabin crew resumed their strike following the failure of talks over pay and staffing levels, grounding flights for tens of thousands of Britons during one of the U.K.’s busiest holidays for air travel.

Flight attendants started a five-day walkout Sunday, forcing the carrier to cancel more than 100 flights from its main base at London’s Heathrow airport, the Unite union said. British Airways says it aims to operate more than 70 percent of long-haul services, compared with 60 percent during last week’s walkout, plus 55 percent of European routes, up from 50 percent.

Last weekend was one of the U.K.’s busiest for travel, with millions of Britons taking a three-day break because of national holiday. Europe’s third-biggest airline is increasing services during the current round of stoppage after what it says is an increase in the number of cabin crew reporting for duty.

Unite says that members might be balloted on action beyond this week’s walkout and another scheduled for June 5.

“It’s an uncomfortable and unsatisfactory situation,” said Jonathan Wober, an analyst at Societe Generale SA in London with a “hold” recommendation on the stock. “My guess is that having come this far BA will stick to their guns.”

Talks between Chief Executive Officer Willie Walsh and the Unite leadership ended without an agreement, the Advisory, Conciliation and Arbitration Service said on May 28. Britain’s state mediator is speaking to both sides to arrange further negotiations to resolve the dispute, spokeswoman Clare Carter said in an interview May 29.

Derek Simpson, Unite’s joint general secretary, told the British Broadcasting Corp.’s Marr program Sunday that Walsh’s “stance” in talks to end the series of strikes is a barrier to a resolution.

“I prefer these negotiations to be in front of a camera,” Simpson said. “If people could see what he is doing, they would know who to blame,” Simpson told BBC.

The strike is costing British Airways 7 million pounds ($10 million) a day, according to an estimate by Unite. The carrier is aiming to save as much as 160 million pounds a year within 10 years by hiring any new cabin crew on less generous wage deals, Chief Financial Officer Keith Williams told investors on May 21.

British Airways fell 1.4 percent to 201.2 pence in London on May 28, valuing the company at 2.3 billion pounds. (Bloomberg)