The outcome of a vote by Ryanair Holdings Plc pilots in the U.K. on whether to strike will be known later Wednesday, adding to the potential for labor strife in the industry during the busy summer travel season.

More than 600 captains and first officers have been voting on union calls for a walkout in Ryanair’s biggest single market after the Irish company failed to propose new pay terms, according to the British Airline Pilots Association.

The threat of travel disruption in the U.K. is building. British Airways will resume its own talks Thursday with Balpa on remuneration for thousands of pilots. Negotiations at the state-backed Advisory, Conciliation and Arbitration Service ended last week with the union agreeing not to set strike dates. Management at London Heathrow, Europe’s busiest airport, made an eleventh-hour pay offer Monday to avert a walkout by as many as 4,000 ground staff.

“We don’t want a dispute in the summer, but it’s not us that has stopped negotiating,” Balpa General Secretary Brian Strutton said of the British Airways dispute, adding that the decision not to set strike dates is intended to avoid escalating the confrontation. The situation at Ryanair is more extreme because the carrier “is not even willing to engage,” he said.

Ryanair didn’t immediately respond to a request for comment. BA said that it was “continuing with talks at ACAS.”

The potential for walkouts is also brewing in other European countries as workers demand a greater share of recent earnings gains. Results from a ballot of Ryanair pilots in Ireland are due Friday, while the carrier will enter talks this week with Spanish flight attendants.

Base Closure

Portugal’s SNPVAC union has also called a five-day strike of Ryanair cabin crew beginning Aug. 21. The labor group said Wednesday that the company had revealed plans to close its base at Faro in the Algarve tourist region as part of a review of operations prompted by the delayed arrival of Boeing Co. 737 Max planes after the model was grounded. Luciana Passo, who heads SNPVAC, said the disclosure may be meant to look like retaliation for the planned walkout, while suggesting that a decision was probably made earlier.

At Ryanair’s Austrian unit, Laudamotion, staff are meeting at Vienna Airport Wednesday to discuss plans for layoffs if unions reject cost cuts and flight-roster changes. Lauda’s stance has reignited the Vida labor group’s campaign for an industry-wide wage deal, which could entail industrial action beyond the airline itself.

Bargaining Power

Ryanair has come under particular pressure after a rostering foul up two years ago increased the bargaining power of pilots and forced it to accept unions. Recognition agreements are in place with ground workers and flight attendants, as well as cockpit crew, across most of its markets, but finalizing pay terms has been problematic as the carrier seeks to maintain a cost edge over rivals.

Pilots at British Airways, the former U.K. flag carrier that’s now the biggest division of IAG SA, say they’ve taken pay cuts twice in the past decade, first during the global economic slump and again when the carrier merged with Iberia in 2011, and expect to be rewarded now that the company is in a healthier position.

Both BA and Ryanair have previously said that they need to keep costs under control as a capacity glut weighs on fares.

Biggest Gain

BA, which on Friday revealed the biggest earnings gain for the June quarter among Europe’s leading carriers, has already agreed to increase pilot pay by 11.5% over three years to an average of about 200,000 pounds ($243,000), up from 183,000 pounds now. Captains on prestigious long-haul services can earn far more. Still in dispute are long-term incentives and profit-sharing terms.

Pilots at Dublin-based Ryanair, which operates only short-haul services and has a flatter pay structure, can earn up to 200,000 euros a year ($224,000).

At Heathrow, the Unite union will put the sweetened pay offer to ground staff, with strikes still planned for Aug. 23 and 24 if they reject the deal. The labor group said it won’t disclose details of the proposal until after the ballot.