O'Leary said that the department of transport had clearly used the government's 25.3% shareholding in the airline to obtain inside information from the company. No other shareholder was informed of the move until the official announcement of the decision six weeks later. He said that Ryanair had no alternative but to lodge an official complaint with the Financial Regulator on Friday regarding this exchange of information because it raised serious regulatory issues.

"You're breaking the law by selectively briefing one shareholder on market sensitive information. Indeed, we'd have refused that sort of briefing if Aer Lingus offered it to us because it's totally against the law, " he said.

He said that the department's argument that the officials had sought the information for policymaking purposes was "bullshit".

"What official policy were those officials working on? Do they even have an official policy on Aer Lingus? I thought their policy was that they had no control over Aer Lingus."

"The department has no regulatory role over airline routes. We wouldn't tell from one decade to the next what routes we open. It's only relationship with Aer Lingus is as a 25.3% shareholder."

Ryanair's complaint was made under market abuse rules, under which individuals can be fined up to 10m and be imprisoned for up to 10 years. The complaint personally identifies Aer Lingus chief executive Dermot Mannion and its chairman John Sharman.

While investigating the complaint, the regulator will have the power to perform "dawn raids" of premises, search homes and seize records.

Aer Lingus has refused to comment on the issue but sources indicated that it would tell investigators that it informed the government of the move as a regulator, not as a shareholder. For its part, the department will argue that its officials sought the information as part of their aviation policymaking duties.

Meanwhile, legal sources have indicated that the case may not be as black and white as O'Leary has claimed, particularly as there was no suggestion that the government had profited from possessing the information.

One expert said that even if Aer Lingus was found to have not treated its shareholders equally, there remained the issue of whether it had breached stock exchange rules on the disclosure of market sensitive information.

The Irish Stock Exchange declined to comment on the issue while the Financial Regulator said that any inquiry would be carried out in accordance with the state's market abuse regulations.