Ryanair pilots have been warned not to sign a letter to airline regulators expressing concern that the airline’s employment practices could jeopardise passenger safety.

In a memo staff were told they would be guilty of “gross misconduct” and “liable for dismissal” if they signed the letter to the Irish Aviation Authority that regulates Ryanair. The letter was drawn up by the Ryanair Pilot Group (RPG), which represents captains and co-pilots working for the airline but is not recognised by the company.

It warned that the “confusing, uncertain and unpredictable employment situation” at Ryanair was becoming “an increasing distraction in daily flight operations”. It added that it was causing “stress and worry” for pilots and had implications for safety.

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Ryanair responded to the letter by warning that any pilot who signed it could be dismissed. “If the Ryanair Pilot Group want to make inaccurate or false claims about non-safety issues they are free to do so, but we will not allow Ryanair’s safety to be defamed by this pilots’ union,” the airline’s chief pilot Ray Conway wrote.

“Please note that any Ryanair pilot who participates in this so-called safety petition will be guilty of gross misconduct and will be liable for dismissal.”

The RPG organised the letter amid concerns that the airline was making the majority of its pilots self-employed. Under the scheme, pilots sign a contract binding them to fly exclusively for Ryanair – but not as employees.

The pilots are then paid for the work they do but have to pay for all their own expenses, including uniforms, identity cards, transport and hotel accommodation. The contracted pilots have no pension scheme or medical insurance unless they set it up themselves.

One Ryanair pilot said that the company was protected because they could claim that pilots had a legal and moral obligation not to fly if they do not think they are capable. But they added: “People are human and if you’re not going to be paid [if you don’t fly] you might think ‘I can do this, I’m fine. I’ll just get on with it’. You should not have a safety culture based on fear.”

David Learmount, operations and safety editor of Flight International magazine and an expert on aviation, said:“Ryanair are pushing their luck on human factors when they employ pilots like a warlord employs mercenaries. There is the worry that if they are self-employed that might place additional pressures on them to work even if, for any number of reasons, they might not feel entirely fit to do so.”

The RPG has now written to the Irish Aviation Authority, which regulates Ryanair, to express its concerns over the memo. “[We are] extremely concerned by some of the rhetoric used in this memo and also by the implicit attempt to constrain the reporting of safety related concerns,” they said in a letter to Kevin Humphreys, Director of Safety Regulation.

“Voicing concerns about safety and petitioning any government agency about those concerns is legal, necessary and in line with every pilot’s legal obligations to report those kinds of concerns when they arise.”

The Independent revealed the poor conditions faced by Ryanair’s cabin crew, who are made to take three months unpaid leave a year, not paid to be on call, and have to pay £360 for their uniform.

In response to the latest allegations, a Ryanair spokesman said: “We do not comment on anonymous, unsigned letters from internet trolls funded by the European Cockpit Association.” He added it was “rubbish” that contract pilots might feel under pressure to work even if they were ill.

“Contract pilots regularly report in sick when they feel they are not fit to fly and we maintain a daily roster for standby pilots for precisely this reason,” he said. “This false claim was previously investigated and dismissed by the Irish Aviation Authority.”