Virgin Atlantic, the UK-based airline, is facing a potential strike by about half of its pilots who argue the airline’s benefits program is unfair. According to a report from Reuters, one of the two pilot’s unions — Professional Pilots Union (PPU) — is currently making initial preparations for a strike vote.

Virgin Atlantic, owned jointly by the Virgin Group and Delta Airlines, has still not negotiated with the PPU, even though more than half of the airlines pilots are members. The rest of the other Virgin pilots belong to the British Airline Pilots Association (BALPA), which, despite having a smaller percentage of Virgin’s workforce, has already signed a deal with the airline.

“[PPU] is threatening strike action unless the company include them in the negotiations and scrap the (benefits) review,” read a statement from the PPU issued Thursday, according to Reuters.

Virgin Atlantic is taking the threat seriously, and a spokeswoman for the airline told reporters in an email that the company is willing to work with the PPU. However, the airline is also aware that the two competing unions present a unique labor challenge.

“We’ve always been willing in principle to recognise the PPU, but we won’t agree to their demand that we derecognise BALPA, and disenfranchise the hundreds of pilots who belong to this union,” the Virgin Atlantic spokeswoman said in an email to Reuters.

Virgin isn’t the first European airline to face a labor crisis this year. According to a report from Reuters, Ryanair, an Ireland-based budget airline finally inked a deal with the German pilot’s union Verdi after a prolonged battle. Although the company opened relations with labor unions in 2017, the company’s unionized cabin crew has fought the company for better pay and working conditions for most of the year.

Although Ryanair and Verdi appear to have come to an understanding, the union members will still need to vote on the initiative to make it official.

“After nearly a year of negotiation, Verdi sees the preliminary agreement as a step toward improving the working conditions of workers and their salaries,” the union wrote in a statement, according to Reuters.