Ryanair's chief executive Mr Michael O'Leary has threatened to sue Aer Rianta unless its chairman, Mr Noel Hanlon, publicly retracts allegations he made concerning the airline at a recent Oireachtas Committee hearing.

Mr O'Leary has written to the chairman of the Joint Committee on Public Enterprise and Transport asking that the allegations be "properly corrected" and the damage to Ryanair's reputation redressed.

In a presentation to the committee on November 18th on the Ryanair proposal to build a £12 million (€15.24 million) terminal building at Dublin Airport in exchange for special bargain facilities including a Shannon facility, Mr Hanlon accused Ryanair of telling "blatant lies".

He said Ryanair had promised to deliver one million extra passengers to Aer Rianta but had only delivered 179,000 passengers.

In a letter to the committee's chairman, Fianna Fail TD Mr Sean Doherty, on November 22nd, Mr O'Leary wanted the "untrue" allegations retracted by Mr Hanlon in front of the committee as the initial allegations had been reported in the national newspapers and on RTE.

"Failing this, Ryanair will have no alternative but to initiate legal proceedings against Aer Rianta for libel so that we may have this untrue accusation laid to rest and Ryanair's good name and reputation restored," states the letter to Mr Doherty, which The Irish Times has seen.

Mr O'Leary said Mr Hanlon also accused Ryanair in his presentation of wishing to design a "cowshed".

"We wish to draw to the committee's attention the fact that the design for Pier D was formulated and finalised in discussion and negotiations between Ryanair and Aer Rianta and is in fact an Aer Rianta design," the letter states.

In another letter to Mr Doherty on November 23rd, Mr O'Leary said he had reviewed the evidence of the chief executive of Aer Rianta, Mr John Burke, at his meeting with the committee on November 4th. Mr O'Leary claimed there were several "untruths" in what he presented to the committee.

"These inaccuracies and untruths go to the very heart of the Ryanair and Aer Rianta submission to the Joint Oireachtas Committee on Public Enterprise," the letter says. "It is imperative to the committee's understanding of this issue that the committee recall Mr Burke and ask him to correct these untruths in his evidence on these specific points."

The letters were circulated to committee members at its meeting last Thursday. Committee vice-chairman, Fianna Fail TD Mr Brendan Daly, said last night a draft reply to Mr O'Leary was agreed.

"The draft reply basically said the matter was between Ryanair and Aer Rianta and it would not be appropriate for the committee to intervene or get involved," Mr Daly said.