Ryanair is facing legal claims from unions for allegedly violating labour laws during a row with striking workers, as the airline faces a new round of industrial action that could affect its low-cost business model.

Ryanair pilots in Belgium, Germany, Ireland, the Netherlands and Sweden are due to hold 24-hour strikes on Friday over terms and conditions. The airline is understood to have cancelled hundreds of flights as a result.

Ryanair faces renewed pressure from unhappy workers after it agreed to recognise trade unions for pilots and cabin crew in December – something the chief executive, Michael O’Leary, had previously said he would rather cut off his hands than agree to. Strikes were partly to blame for profits falling in the first quarter of its financial year.

But after making deals with unions in countries including the UK and Italy, the airline has failed to engage with workers, unions claim. Pilots and cabin crew have responded with a string of strikes seeking better terms and conditions. Cabin staff in Belgium, Italy, Portugal and Spain walked out for 48 hours last month.

Ryanair responded to strikes in Ireland by issuing redundancy notices to 300 staff in Dublin last month and shifting some planes from Ireland to Poland. Soon afterwards, O’Leary said: “If we have people who just want to have strikes for the sake of having strikes then they can have strikes and they’ll find themselves [with] jobs getting moved and aircraft getting moved.”

Unions have alleged Ryanair is using other tactics to deter workers from striking.

While most EU countries allow companies to dock pay from striking staff, union sources said Ryanair threatened to strip crew of productivity bonuses and warned that their promotion chances would be affected, which would be in breach of labour laws.

In a letter from Ryanair’s chief people officer, Eddie Wilson, to Spanish crew, the airline promised that “no one can be punished or victimised” if they chose to strike. But one cabin crew member showed the Guardian a subsequent letter that said they would lose their monthly productivity bonus.

The letter continued: “All no-shows are recorded in your absence record and are taken into consideration along with all relevant factors of performance when assessing promotion and transfer opportunities.”

The Independent Transport Workers Federation (ITF), an umbrella body for trade unions, said Ryanair could be hit with lawsuits for allegedly breaching labour laws in several countries.

“Unions in countries directly affected by all the latest actions against Ryanair workers are pursuing a mixture of legal claims and investigations with the appropriate government agencies,” said ITF’s general secretary, Stephen Cotton. “They are having to claim once again that Ryanair are infringing their members’ fundamental rights in breach of labour laws in their country.”

A Ryanair spokesperson said: “As with any company dealing with industrial relations disputes and strike action, employee attendance is recorded so that pay can be adjusted accordingly. Participation in a strike does not affect promotion or transfer decisions and this was also confirmed to our crew.”

O’Leary has said he is willing to deal with short-term disruption rather than give in to demands that undermine the low-fare, low-cost business model that has made Ryanair Europe’s biggest airline.

But investors are showing signs of doubt. Ryanair shares have lost a fifth of their value in the past year while its low-cost rival easyJet, which has long-standing union agreements, saw its share price rise by a fifth.

Most equity analysts argue Ryanair will be able to get through the turmoil. Gerald Khoo at broker Liberum said: “There is space for Ryanair management to reach sensible agreements so that Ryanair will have the same profitability in the long term.”

HSBC aviation analyst Andrew Lobbenderg argues that Ryanair’s industrial relations problems call into question its low-cost model and its governance. Its troubles may lie partly in the lack of challenge by board members to their “high-profile, charismatic CEO”, Lobbenberg said.

“We foresee higher wages, social costs, reduced productivity, flexibility and higher overhead costs,” Lobbenberg wrote in a note to investors. “The evolution towards employment practices more in line with industry norms will structurally weaken profitability and thus poses deeper questions to management and the board.”

Until recently, Ryanair was able to divide its workforce by striking different deals across the countries it flies between. Containing the strikes may be difficult for a company whose historic failure to recognise unions leaves it with little experience to draw on for a major dispute.

Tony Dundon, professor of human resource management and labour relations at Manchester University, said: “The idea of riding it out is typical of a small entrepreneurial mindset rather than a more sophisticated management process. The key problem is the potential for escalation.”