Ryanair has won a High Court order overturning a decision of the Commission for Aviation Regulations concerning the manner in which landing and take-off "slots" for airlines using Dublin airport should be allocated.

Mr Justice Kevin O'Higgins overturned the commission's decision of April 2005, which altered the designation of Dublin airport as an airport where slots were "schedule-facilitated" to an airport where slots were "co-ordinated".

He ruled the regulator's decision was in excess of its powers and failed to conform with an EU regulation on the management and operation of airport slots.

A "scheduled" airport is one where the various requirements of airlines operating at the airport can be met through a broad measure of consensus as to how the landing and take-off slots should be allocated, while a "co-ordinated" airport is one where slots are allocated by a facilitator independent of the managing body of the airport.

Ryanair had argued for a "schedule-facilitated" rather than a "co-ordinated" regime because this offered it greater flexibility.

Dublin airport was designated a "schedule-facilitated" airport in 2000, but Aer Rianta, now the Dublin Airport Authority, had requested in 2002 that it be redesignated as a "co-ordinated" airport. A report on the matter in July 2004 recommended that the then operable system of voluntary co-ordination continue for another three years.

However, the regulator had in April 2005 announced a decision to designate the airport as "co-ordinated", Mr Justice O'Higgins said. The commission, he noted, said this was in the context of a very significant rise - about 100 per cent - in the scale of refusals to co-operate with the voluntary system.

It also said Ryanair itself was responsible for a large proportion of those refusals.

In his view, there was a need for an updated capacity analysis before any redesignation of the airport, and the commission had acted in excess of its powers, the judge found.

He noted the commission had argued that quashing its decision would lead to serious consequences, additional difficulties and possibly more congestion in the airport. However, such difficulties had not been the subject of detailed analysis in the court, he said.

While he was prepared to accept that the quashing of the commission's order would pose difficulties and cause inconvenience, he was not satisfied those consequences constituted adequate grounds for refusing relief to Ryanair.

The judge noted the commission also argued that the quashing of its decision risked being "moot" having regard to the fact that it was anticipated that Dublin airport would be changed to co-ordinated status in the scheduling period from summer 2007 .

While the designation of the airport may be changed to co-ordinated status in 2007, it would be wrong for the court to act on the presumption that that change in designation would take place, he said.

The possible future designation was not a ground for refusing relief to Ryanair, he ruled.