Ryanair is planning to take legal action against the Commission for Aviation Regulation after it announced yesterday that it plans to make Dublin a "co-ordinated" airport, a move aimed at regulating competition for aircraft slots at peak times.

Cathal Guiomard, the aviation regulator, yesterday decided that aircraft slots at Dublin airport should be co-ordinated from March 25th to October 27th. The regulator said peak demand at Dublin airport this summer would be greater than the airport's capacity, which would lead to "significant delays".

These problems could not be resolved in the short term, he added. The allocation of slots will be managed by Airport Co-ordination, a British company that carries out similar tasks at Heathrow, Gatwick and Stansted airports.

To date, a voluntary system has been managed by Airport Co-ordination. Some airlines, including Ryanair and Aer Lingus, have chosen to ignore their allocations and to schedule flights according to their own preferences. "It's been a case of survival of the fittest at peak times," said one senior industry source.

The regulator's decision was instantly challenged by Ryanair, which said it would take its case to the High Court. The low-fares airline successfully blocked Mr Guiomard's attempt to make Dublin co-ordinated in 2005 when the High Court ruled the regulator's consultation process was flawed.

"The commission's decision to slot co-ordinate Dublin airport makes even less sense this time than the last time around," said Jim Callaghan, Ryanair's head of regulatory affairs. "There is more check-in capacity this year with 25 additional check-in desks and 14 more stands coming on stream in winter 2007 with pier D [set to open].

"Runway capacity is also clearly not being fully utilised as Dublin already has two runways and yet handles only half the 40 million traffic of the one runway at Gatwick airport."

Ryanair disputes the view that Dublin is congested, arguing that it could not have added nine aircraft and three million extra passengers since last April if that was the case.

The regulator's decision is a potential blow to Ryanair, which plans to base five additional aircraft in Dublin this year. When considering what slots to give airlines, Airport Co-ordination is likely to give a greater weighting to those who have been flying routes for some time. This could work against Ryanair and its plans for new services.

Aer Lingus said it was "broadly supportive" of the regulator's decision. The Dublin Airport Authority also welcomed the decision.