The union representing Ryanair pilots in the UK has threatened strike action if the airline refuses to negotiate a seniority system for promotions and transfers.

The threat comes in a letter issued last Thursday by the British Airline Pilots’ Association (BALPA) to Ryanair Chief Executive Michael O'Leary.

BALPA had given the airline until today to respond.

BALPA National Officer Terry Brandon said he wrote to Ryanair "explaining that pilots urgently want to negotiate an agreement on seniority and pointing out that we would consider an industrial action ballot if Ryanair did not agree to talks.

"I can now confirm that we have a meeting with the company next month when pilot seniority will be on the agenda for discussion."

Ryanair told RTÉ News that it does not comment on negotiations with its people.

BALPA was the first pilots' body to agree a recognition agreement with Ryanair following the company's shock decision before Christmas to recognise unions.

In the letter dated 24 May, Mr Brandon confirmed that they had been in discussion with the airline on a number of issues, including allocation of annual leave and base transfers that relate to, and are dependent on, establishing an agreed seniority system.

However, he said that negotiation of an appropriate and agreed seniority system is now a "first priority" for BALPA, as it would provide transparency and fairness to decisions made by management that can have a very large impact on the lives of pilots and their families.

He went on to say that Ryanair pilots complain that there is no transparent system for the determination of important matters, including voluntary or involuntary base transfer or allocation, command upgrades, promotions, and the allocation of annual leave.

Mr Brandon noted: "When a pilot receives notice of a mandatory base change or is denied a request for a change of base, such management decisions can have a devastating effect on family life.

"It is self-evident that the transfer to another base can cause family upheaval and, in some cases, cause family breakdown."

He stated that the current system for promotion from First Officer to Captain in most cases includes a mandatory change of base, despite potential opportunities for captains in their existing base.

Mr Brandon claimed that because of the "profound" consequences a base change can have for pilots' families, some First Officers may not put their names forward for promotion, resulting in a "stunting" of career progression.

He said the stress on the pilot may be further compounded by the absence of a transparent selection mechanism, or an objective mechanism to review such decisions.

Mr Brandon set out proposals for a seniority agreement, which he said would assist the airline in recruiting and retaining pilots it needs for future growth and success.

He warned: "This issue is of sufficient importance that 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."

Separately, unions representing Ryanair cabin crew, who have already mounted three days of strike action in Portugal, have warned of possible disruption to summer travel if Ryanair does not "comply with national labour laws" by 30 June.

Representatives of Spanish, Portuguese, Italian and Belgian cabin crew unions met in Madrid yesterday to discuss the legal requirements in each country for industrial action, as well as a response to disciplinary measures imposed on cabin crew from other countries who refused to "break the strike" last Easter.

Ryanair has always insisted that its staff were employed under Irish law, but unions insist the rules of the country where they are actually based should apply.