Irish-based Ryanair pilots have served notice of industrial action in their dispute over pay and working conditions.
The Fórsa trade union said 94% of the pilots backed the action.
The union is required to give the airline at least one week’s notice. It said it will outline its plans within seven days unless the airline agrees to union pay proposals by this coming Monday.
Their UK-based colleagues have already announced a total of five strike days at the end of this month and the start of September.
Fórsa assistant general secretary Ian McDonnell said the strike action could be avoided if the airline “engaged professionally and constructively in talks.”
He argued: “Ryanair’s directly-employed Irish-based pilots are simply seeking pay levels that are common and competitive in the commercial airline sector, from a company that made a more-than-healthy profit of €1 billion last year.
“They feel they have been forced into contemplating potentially-disruptive industrial action by a company that seems either unwilling or unable to negotiate in a professional and constructive manner.
“At this stage, only a substantive counter-proposal, which properly addresses all areas of our claim, will be enough to prevent us serving notice of industrial action next week.”
Responding to the results of the ballot, Ryanair said it was "disappointed" with the vote.
They claimed the union had failed to provide the airline or mediator Kieran Mulvey with 'specific pay proposals'.
The company said in a statement: "This Fórsa threat of industrial action has no mandate from Ryanair pilots, and is ill-timed just 10 weeks before a hard Brexit, while Ryanair is in the process of letting excess pilots go due to the Boeing MAX delivery delays.
"Fórsa should continue in mediation and avoid threatening unnecessary disruptions to customers holidays and travel plans."
The union said it submitted a pay claim to Ryanair in late March seeking pay levels and structures that are in line with sector norms."
It accused the airline of using "stalling tactics" during negotiations - by seeking costings of the claim and then failing to provide the operational information required to deliver them.
The airline has yet to make a counter-offer.
The UK dispute centres on a range of issues including pensions, loss of license insurance, maternity benefits, allowances and pay.
Pilots have insisted they do not wish to disrupt anyone’s travel plans – and said they have no choice but to strike.
The strikes could result in thousands of people having their flights cancelled; however, the airline has insisted it will do its utmost to avoid disruption.
Meanwhile, the 180 Irish-based Ryanair pilots who voted on the action are members of the Irish Airline Pilots Association (IALPA), which is a branch of Fórsa.
Ryanair has urged UK pilots to return to talks and has apologised to customers.
The industrial disputes come just a week after Ryanair announced a 21% fall in profits and said up to 900 jobs were at risk.