Members of Ryanair's British pilot union voted to strike in a dispute over working conditions including pay, pensions and benefits, the British Airline Pilots Association (BALPA) said on Wednesday.
The union said there would be a strike from August 22-23 and one from September 2-4.
"The ballot saw 80% vote in favour of strike action on a 72% turnout," BALPA said in a statement. Ryanair said on Wednesday that less than 50% of its pilots were members of BALPA.
Last year, pilot strikes led to hundreds of flight cancellations across Europe.
"We are disappointed that the pilots union BALPA, is threatening to disrupt our customers travel plans during late August, early September," Ryanair said in a statement.
Ryanair announced plans last month to cut hubs and flights starting in November after delivery delays of Boeing's 737 MAX planes. Boeing's plane was grounded after two crashes killed a total of 346 people.
Ryanair said the delays would cut the airline's expected growth rate next summer from 7% to 3%.