Ryanair cabin crew have said they are considering the biggest strikes in the airline’s history unless it agrees to improved working conditions.

The Dublin-based airline suffered its worst ever strikes this summer, as action by pilots and cabin crew forced it to cancel flights, including to major holiday destinations such as Italy, Portugal and Spain.

It secured a breakthrough in August after reaching a deal with striking Irish pilots and said it was hopeful it could secure deals in other markets. But on Friday unions representing cabin crew in Italy, Portugal, Belgium, Spain and the Netherlands threatened in a joint letter to extend the summer of industrial action into the autumn.

Unions locked in a long-running battle with the airline said they would hold “the biggest strike action the company has ever seen” unless changes were made to their conditions. After a meeting in Rome, unions demanded that staff were given local contracts under local law rather than Irish contracts, which Ryanair uses across its European workforce.

They said the strike would take place in the last week of September and that a decision on whether to go ahead would be made by 13 September.

“We want to solve this. We don’t want to strike,” said Fernando Gandra, director at Portuguese union SNPVAC. He said the two Italian unions also had pilot members and that the unions would invite pilots and ground staff across Europe to join their cabin crew colleagues in the strike.

Ryanair, which says its staff have better conditions than many low-cost rivals, did not immediately respond to a request for comment. The airline confirmed earlier on Friday that it would reverse a decision made in July to move 20% of its Dublin-based fleet to Poland, due in part to the damage strike action was doing to bookings.

“The board and management of Ryanair are committed to union recognition, and working constructively with our people and their unions to address their reasonable concerns, as long as this does not alter Ryanair’s low-cost model or our ability to offer low fares to our customers,” Ryanair’s chief people officer Eddie Wilson said in a statement on the Dublin fleet plans.

The airline said it had informed 300 pilots and cabin crew in Dublin that their jobs were now safe.