Ryanair pilots have formed an unofficial trade union and are invoking the spirit of Winston Churchill in a battle with chief executive Michael O’Leary over their employment terms.

O’Leary has clashed with pilots amid the fallout from the Dublin-based carrier’s recent pilot rostering “mess-up”, which saw more than 700,000 passengers affected by flight cancellations.

A letter circulated among pilots and seen by the Guardian indicates they are coordinating to derail Ryanair’s strategy of negotiating separately with multiple employee representative committees (ERCs), small groups representing individual airport bases.

Instead, they have now formed a central committee to replace the ERCs, in a move designed to improve their bargaining power.

According to a letter from the committee to pilots, it was set up to “oppose the long standing strategy of divide and conquer that Ryanair has applied in its dealing with pilots”.

It warns that pilots are likely to face a tough battle given Ryanair’s policy of not recognising trade unions and dealing instead with multiple ERCs separately. Ryanair allows its employees to be members of a trade union, but it generally refuses to recognise or negotiate with unions.

“To overthrow this structure will require immense force of will, stamina and commitment from every pilot,” the letter says.

“It is entirely possible that things will get worse before they get any better. Expect management to fight any change by all means available to them. There will be casualties.”

In a final rallying cry, the committee urged pilots to make autumn 2017 “the beginning of the end of the race to the bottom for our profession”.

“To paraphrase Churchill: Let us make sure future colleagues look back and say ‘This was their finest hour.’”

According to the letter, while Ryanair has offered improved pay terms to pilots at four bases – Stansted, Dublin, Frankfurt and Berlin – none has yet agreed to any deal.

It said Stansted pilots are in the process of electing new representatives, while other UK airport bases have arranged meetings with British pilots’ trade union Balpa to take place on Monday and on Wednesday.

Pilots in Dublin have agreed to approach Irish pilots’ union IALPA with a view to some form of cooperation, the letter also claimed.

It urged pilots at individual airport bases not to respond to any offers from the company and to report any such communication to the committee.

Committee members will remain anonymous until they have secure legal guarantees that there will be no reprisal from Ryanair for their actions.

Despite the letter’s confrontational tone, the central committee warned pilots to think carefully before adopting tactics such as co-ordinating mass sick days or “work to rule”, which would involve refusing to carry out non-contractual tasks such as turning up early to ensure punctuality.

The committee warned this could expose individual pilots to lawsuits from the company, or trigger disciplinary procedures that could cow other pilots into silence.

Rather than making detailed demands, the committee said it was seeking a structural overhaul of pilots’ representation at Ryanair, including legal tools that would allow them to push for changes to employment terms.

Ryanair had not responded to a request for comment at the time of publicaiton.

Separately, the airline cancelled a further 21 flights from Pisa due to a strike by ground handling staff at the Italian airport.

It also repeated its apology to more than 700,000 customers affected by two waves of flight cancellations due to its pilot rostering error.

Ryanair said alternative routings or refunds had been processed for 98% of passengers affected by the first wave, while 75% of those affected by the second wave have now been assisted.