Airline faces fresh trouble as pilots turn down bonus offer to work on holidays and starting early.

Ryanair customers face the threat of a fresh wave of flight cancellations as the airline’s pilots prepare to reject an offer of a cash bonus if they give up days off.

The Guardian has obtained a draft letter signed by Ryanair pilots from across Europe, rejecting the offer and warning they will now “work to rule” – refusing to work beyond their basic contractual obligations. Ryanair had told pilots earlier this week that if they declined the £12,000 payment more flights might have to be scrapped.

The no-frills carrier is scrambling to cope with a public relations disaster after it announced plans to cancel up to 50 flights a day until 31 October, citing a “mess-up” in how it schedules time off for pilots. The move has affected 315,000 customers.

The letter circulating at Ryanair bases across Europe states: “We would like to advise that with immediate effect the pilot workforce at the bases [airports] listed below rescind the goodwill that has been extended toward the company for many years,including working days off and turning up early. In short, we shall now ‘work to rule’.”

Working to rule would mean pilots refusing to help the airline by going beyond the terms of their contracts, which would involve working days off, arriving for shifts early or even answering company calls when they are not at work.

The pilots have been spurred on by colleagues flying for rival European airlines, amid concern that Ryanair is leading a “race to the bottom” that has seen terms and conditions watered down across the industry.

The draft letter is a response to Ryanair’s chief operations officer, Michael Hickey, who this week offered a tax free bonus of up to £12,000 to pilots willing to give up time off in an effort “to avoid further cancellations”.

The letter takes issue with Hickey’s offer on six separate grounds, including that pilots must have worked 800 flight hours in a year to be eligible for the tax-free bonus.

“Pilots have checked their logbooks and many have never achieved 800 flight hours in a single 12-month period ever,” it says.

The pilots also object to “ambiguity” surrounding the offer’s conditions, including the number of days off they would be required to work to get the bonus and whether enough days would even be allocated by Ryanair.

Ryanair pilots at the airline’s bases across Europe are discussing whether to put their names to the letter, but several are understood to have done so already.

The airline’s 4,200 pilots – many of whom work as agency contractors – are not unionised, and could face the risk of being fired for taking such action.

But one pilot familiar with the discussions said the airline, which has seen more than 700 pilots quit in the past year, is too short of experience to take such drastic action.

“They are not in a position to fire pilots because it would exacerbate their problem,” they said.

The letter also makes reference to pilots’ strong bargaining position, saying: “The company faces a pilot shortage and even if this is only for a short period of time, the reputational damage that could result […] could be extremely long lasting both in terms of customer and shareholder confidence.”

Ryanair pilots are being encouraged and offered advice by their peers at airlines across Europe.

“There has been a race to the bottom in terms and conditions which is led by Ryanair and it’s having an impact throughout Europe,” the pilot said.

“It’s sucking guys out of Europe to the Middle East and China where terms and pay are better.”

Pilots are expected to demand that Ryanair agree to offer them local contracts offering full employee rights under the employment law in the country where they are based.

“The continued erosion of the pilots terms and conditions over the past 10 to 15 years has to be not only stopped but reversed,” the letter says.

Ryanair has not yet returned a request for comment.

The airline said on Wednesday that it hoped to have made new arrangements for more than 175,00 customers on other Ryanair flights and processed 63,000 refunds, taking on extra customer service staff to deal with the problem.

But consumer group Which? said the information it had offered to passengers about compensation “falls woefully short”.

Alex Neill, Which? managing director of home products and services, said: “It [Ryanair] is legally required to spell out compensation rules when a flight is cancelled and, in our view, have so far failed to do that, leaving passengers hunting around for information.

“This is another blow for the thousands of passengers who have already had to endure huge inconvenience as a result of this fiasco.

“The airline must now automatically compensate eligible passengers without them having to go through the additional hassle of making a claim.”