Ryanair in December recognized trade unions for the first time in its 32-year history, but has since then struggled to reach agreement on the terms of the recognition with some unions and it faces several threats of strike action.

Industrial action in Britain, Ryanair's first market to agree a formal union recognition agreement, could be particularly damaging. The airline has experienced minor disruption due to industrial action in Germany and Portugal, but has so far avoided a major stoppage.

Ryanair's Irish union two weeks ago threatened possible strike action unless the airline agrees to new working practices.

In the letter to Chief Executive Michael O'Leary, dated May 24, the BALPA union demanded a seniority system to "provide transparency and fairness to decisions made by management which have a very large impact on the lives of our members and their families."

Ryanair and BALPA representatives are due to meet to discuss the issue in June, a BALPA spokeswoman confirmed on Wednesday.

The union has said it wants transparent systems for involuntary base transfers, command upgrades and the allocation of annual leave and promotion.

"If it is not possible to immediately negotiate the introduction of such a Seniority Agreement, it is our intention to escalate this matter as a matter of urgency, to seek sanction for industrial action up to and including strike action should that be necessary," the letter said.

The letter requested a positive response to the letter "on or before" Tuesday, May 29.

A spokesman for Ryanair said the airline did not comment on negotiations with its staff.

(Reporting by Conor Humphries, additional reporting by Sarah Young; Editing by David Holmes and Mark Potter)

By Conor Humphries