Ryanair cabin crew have been told they could face “disciplinary proceedings” and have their working hours forcibly changed unless they sell more perfume and scratchcards.

The Irish airline has previously denied pressuring staff to hit specific sales targets, after it emerged they were encouraged to sell products in return for bonuses.

But letters sent to crew members by recruitment firms that supply staff to Ryanair – seen by the Guardian – warn of dire consequences for those whose average sales per flight fall “below budget”.

The letters highlight 10 products, including drinks, confectionery, cosmetics and scratchcards, listing the percentage of flights in which individual cabin crew members had not sold enough.

Cabin crew were also criticised for too often failing to sell more than €50 (£45) of merchandise, indicating a fixed sales target.

The letters, one from recruitment agency WorkForce International Contractors and another from an agency that cannot be named because it could not be reached for comment, feature almost identical wording.

“This performance is not acceptable and it is clear that you are simply not doing your job on board,” the letters state, warning crew members they had “drastically underperformed”.

Crew members were also told that their sales were being “closely monitored” and that if they did not sell more goods “further action will be taken and you may be subject to disciplinary proceedings”.

Ryanair’s operational manual for cabin crew states that their function is “primarily for the safety of passengers”.

Earlier this year the airline denied putting any “pressure” on staff to make sales after a separate memo was leaked instructing cabin crew to sell a fixed number of meals, drinks and scratchcards or explain why they had not.

But as well as the threat of “disciplinary proceedings”, the new letters show that crew were told that Ryanair could force staff who don’t hit targets to plug staffing gaps at short notice, rather than enjoying fixed working hours.

In 2015, Ryanair agreed a new pay and rostering deal with agency staff, under which they work for five days and have three days off.

But cabin crew were told Ryanair had “no obligation” to provide this roster if sales targets were not met and that, as of next month, they could be called to work at arbitrary times on a weekly basis.

“Our client airline [Ryanair] will continue to offer favorable [sic] conditions including pay and roster agreements to our cabin crew, however this can only apply to those crew who contribute,” the letters said.

The fixed rostering pattern would be restored if crew members increased their sales performance, crew were told, but not until next year.

The Irish trade union Impact, which was shown a copy of one letter, said: “It makes for grim reading.

“The primary role of cabin crew is flight safety, in-flight sales is a secondary role.

“This correspondence suggests a crude approach to performance management, and reveals the vulnerability of individual staff if they don’t reach the targets they’ve been set.

“The client airline’s position on organised labour is well documented, so I’m struck by the fact, more than anything else, that this person had nobody to turn to when they received that letter.”

The sale of food, drink, perfume and other items such as scratchcards is a major part of Ryanair’s income, providing £1.5bn in “ancillary revenue” last year.

The figure makes up 27% of the airline’s total revenue and it said last year that it wants to raise that proportion to 30%.

Ryanair said it had not seen the memo but added: “While we cannot comment in detail on the WorkForce letter, it clearly does not set any ‘targets’ which must be met.

“As the letter makes clear, any individual, who consistently, and markedly, underperforms, may face disciplinary proceedings ‘if there is no significant and sustained improvement.’

“We expect underperformers to improve, and they are given time and training to do so. What does the Guardian do with its underperformers?”

WorkForce did not return requests for comment.

Members of a private Facebook group for Ryanair cabin crew, who asked not to be named for fear of reprisal, expressed their dismay at the tone of the letter.

One branded the carrier a “bunch of bullies” while another described the difficulty that cabin crew face in improving their working conditions.

“They don’t care if you leave the company, for each one of us leaving they can replace with 20 new young guys coming from school and working as they want.”

Ryanair’s employment practices have come under increasing scrutiny of late amid a row with pilots that erupted following mass cancellations caused by the company’s failure to roster its staff properly.

The dispute saw pilots form an unofficial trade union in a battle with the chief executive, Michael O’Leary over their employment terms.

It has also emerged that Ryanair pilots are being investigated by HM Revenue & Customs over complex employment structures imposed on them by the no-frills airline.