Ryanair flights to and from Dublin could be disrupted by strikes over Christmas after pilots based in Ireland voted for industrial action.

Europe’s biggest short-haul airline now faces disruption in key European markets following ballots for action by pilots in Italy and Portugal over the past week.

A vote by members of the Irish Ialpa union resulted in 79 of 84 voters backing action.

While Ialpa represents only some of Ryanair’s Dublin pilots, many of whom are employed via third-party agencies, those who voted are mostly captains, without whom planes cannot fly. However, it has not been decided whether the industrial action will include strikes. The executive committee of Impact, an umbrella union to which Ialpa is affiliated, is due to meet on Tuesday to discuss what shape the protest will take.

The ballot was held against the backdrop of increasingly sour relations between Ryanair and its pilots, many of whom are seeking collective bargaining power via a new European employee representative committee (ERC).

Ryanair usually deals with multiple European ERCs separately, allowing it to negotiate separate deals on issues such as pay and conditions at 87 different bases.

In a letter sent to Ryanair’s chief people officer, Eddie Wilson, on Sunday, the company’s Dublin ERC said this form of industrial relations had failed and condemned a threat to punish striking pilots with cuts to pay and benefits. In the letter, seen by the Guardian, pilots told Wilson that the warning “only proves that respectful and equal negotiations are not possible. This is simply not acceptable behaviour and this counter-productive engagement should stop immediately”.

Ryanair said it had received no notification of any industrial action, calling Ialpa an “Aer Lingus pilots group”, referring to a rival Dublin-based carrier.

“Now that Ryanair’s Cork, Shannon and Belfast bases have agreed 20% pay increases, we expect the Dublin pilots to follow this trend,” it said. “However, if Ryanair’s Dublin pilots are misled by some Aer Lingus pilots into industrial action then they will lose their favourable rosters [...] and remuneration benefits that are specifically linked by agreement to dealing directly with Ryanair. If any such industrial action occurs Ryanair will still not engage with, or recognise, Aer Lingus pilots or their Ialpa union.”

Ryanair has come under increasing pressure from pilots since it announced two waves of cancellations due to an error in scheduling their hours. Pilots are understood to have been emboldened by a belief that the airline is still short of qualified captains and first officers. Ryanair has argued that it is recruiting pilots and there is no threat of a shortage.

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