The Minister for Transport, Martin Cullen, has intervened in the controversy over the terminal at Dublin Airport by writing to aviation regulator Bill Prasifka and emphasising the importance of developing the airport.

The Minister has decided to avail of powers given to him under Section 10 of the Aviation Regulation Act, which entitles him to give general policy guidance to Mr Prasifka. The move may stir controversy at a time when the regulator is preparing to make a key decision on the passenger charges regime at Dublin Airport. This decision is expected in the next fortnight.

In the letter, written in mid-August, he urges Mr Prasifka to bear in mind the importance of upgrading the infrastructure at Dublin Airport, which he says is important for the wider economy and passengers generally.

The intervention of Mr Cullen may prove controversial, although the Minister is acting within his powers. In 2001, the then minister for public enterprise, Mary O'Rourke, issued general policy guidance to the regulator. In that instance, the advice concerned regional development.

The letter from Mr Cullen affirms the broad independence of Mr Prasifka's office, but strongly emphasises that the Government would like to see a decision on passenger charges at Dublin Airport made as soon as possible. However, the letter does not make any specific recommendation about what decision Mr Prasifka should make.

This week, the chairman of the Dublin Airport Authority (DAA) Gary McGann said he wanted a €2.50 increase in passenger charges at Dublin.

He said without this increase, the investment in a new terminal could not go ahead. Depending on design specifications, the terminal, located in the southern part of the airport, will cost between €170 million and €200 million. This week, Ryanair and the DAA clashed over the level of consultation undertaken with airlines concerning the terminal.

Ryanair said it had not been fully informed of the plans, but the DAA said all attempts to brief Ryanair had been made.

At this point most observers believe Aer Lingus and some of the main US carriers will be housed in the new terminal. This is because it would be easier to handle wide-bodied, long-haul aircraft in the new terminal. Ironically, this could leave Ryanair occupying the older terminal's facilities. Its chief executive has long described Dublin Airport as the "black hole of Calcutta".