Pilots and flight attendants plan to strike for four hours from 1 p.m. local time on Dec. 15, the Anpac union said in a letter to the airline, adding that they’re campaigning for the right to negotiate collective labor agreements, as well as on issues including social security, health care and vacation planning. The FIT-CISL union said separately that the action had been deferred from Dec. 10 at the request of the Italian transport ministry.

Pilots in Ireland, Spain, Germany, Sweden and Portugal, as well as Italy, have formed local labor councils in the past few weeks in order to press for collective bargaining outside of Ryanair’s in-house employee representative committees. The move also opened up the possibility of strike action.

Ryanair has been left vulnerable after a staffing mix-up required it to squeeze six months of pilot leave into half that time. The crisis was exacerbated by the poaching of crew by other airlines, forcing the Dublin-based carrier to cancel more than 20,000 flights and propose a new pay deal in order to maintain other services. The take-up of the offer has been slow as some staff seek to exploit the opportunity to boost their leverage with the company.

Ryanair said that the Anpac and FIT-CISL unions represent employees of Italian carrier Alitalia SpA and have “no role” at the Irish carrier, having threatened walkouts five times in the past only to call them off.

“We expect this latest threatened strike will also be postponed or canceled,” spokesman Robin Kiely said.