Ryanair cabin crew have reportedly been urged to sell more on-board merchandise or they could be subject to "disciplinary proceedings" and having to fill rostering gaps at short notice.

The recruitment firms that supply staff to the Irish carrier have sent letters to crew members in which they state that those whose sales fall below average could face consequences. The letter, which was subsequently seen by The Guardian, listed 10 items sold on board, including scratchcards, drinks, perfumes and confectionery, and highlighted the percentage of flights for which each cabin crew member had not sold enough.

"This performance is not acceptable and it is clear that you are simply not doing your job on board," the letters state, spelling out to crew members they had "drastically underperformed". Staff were also warned sales were being "closely monitored" and that if they did meet their targets "further action will be taken and you may be subject to disciplinary proceedings".

The letter also stated that staff who do not meet their sales targets could be asked to plug rostering gaps at short notice, in stark contrast with the policy the airline established in 2015, when it agreed a new pay and rostering deal with agency staff.

The agreement guarantees agency staff a fixed roster, which sees them working five days and having three days off. However the airline appears to have now changed its policy, with the latter highlighting Ryanair "had no obligation" to provide this roster if sales were not met.

From December this year, the letter added, staff could be summoned for the weekend during the week on an arbitrary 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," it said.

The airline, Europe's largest carrier, defended itself by saying the letter did not set any targets.

"While we cannot comment in detail on the letter, it clearly does not set any 'targets' which must be met," the airline was quoted as saying.

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

Earlier this year, the Irish airline came under scrutiny after a memo instructing cabin crew to sell a certain number of food and drinks or explain why they had not done so was leaked to the press. However, the Dublin-based carrier dismissed the allegations, insisting it had never put its staff under any pressure to meet sales targets.

The sale of food, drinks and other merchandise plays a major role in Ryanair's income, accounting for 27% of total revenue in the last financial year, a figure the airline expects to rise to 30% over the next 12 months.

The news will bring more unwanted scrutiny onto the airline, which in September was forced to cancel approximately 20,000 flights affecting some 700,000 passengers. The issue stemmed from its decision to reschedule its holiday year to run from January to December, rather than the current system, when it runs from April to March. As a result, it had to allocate annual leave to pilots in September and October.

The blunder cost the airline €25m in compensation costs and forced it to hike the salary of its pilots, which the airline claims is now 20% higher than its rival Norwegian. The new wage structure , however, will result in a €100m increase in costs this year for the airline, which has recruited 900 pilots so far this year.