The news comes as sources close to the commission indicated that it believed either the airline or the department of transport were behind the long-running series of leaks, which seemed designed to undermine Ryanair's bid.

"There are parties involved in this case who may have had a strong interest to leak and, if I was a gambler, I'd be betting on the department and Aer Lingus, " said a source.

Meanwhile, Ryanair chief executive Michael O'Leary confirmed that it had complained that Aer Lingus revealed details of concessions it made to the Commission in its takeover defence document published last December.

"They were meant to have the documentation under the process but they were legally banned from publishing it but they included it in one of their defence documents, " he said.

It is understood Ryanair's complaint claims that Aer Lingus published the material despite signing a confidentiality agreement with the Commission 24 hours previously.

It also claims that Aer Lingus was behind press reports quoting "a source close to the deal" who stated that its concessions were "meaningless in some cases, undeliverable in others or irrelevant".

It is understood that the complaint states that these comments came at a sensitive point in Ryanair's bid and were damaging to the low-cost airline.

Ryanair also says that the commission failed to take any action against Aer Lingus despite repeated complaints. It subsequently requested that no further confidential information be sent to Aer Lingus but its request was ignored.

The airline also claims that the commission itself was also responsible for other leaks, including the disclosure of its statement of objections to bid to news agency Bloomberg in March.

Todd said the commission conducted a thorough internal investigation into the leaks and had concluded that they came from outside the commission. He refused to comment on where the commission believed the leaks had come from. Meanwhile, an Aer Lingus spokesman denied that it had leaked any confidential documentation.

"The information contained in the Aer Lingus circular published on 1 December was not drawn from any confidential documentation but referred to information that was already available from other sources, " he said.

A spokeswoman for the Department of Transport also said that it was not responsible for any of the leaks referred to in Ryanair's complaint.

While the EU Ombudsman lacks any binding legal powers, Ryanair's head of regulatory affairs, Jim Callaghan said that the airline had lodged the complaint to highlight its belief that the Commission and Aer Lingus had been leaking sensitive information to the press during the competition investigation.

"In a sense, these incessant leaks seriously undermined the whole process and highly confidential information supplied by Ryanair to the Commission was leaked to the market. It's unbelievable, " he said.

However, the Commission's competition spokesman Jonathan Todd said that the Commission had been powerless to stop the leaks.

"We don't have any sanctions we can apply. That's the problem. Normally, the lawyers who deal with these things are bound by codes of conduct and rules governing confidentiality, " he said.

Todd said that the Commission had conducted a thorough internal investigation into the leaks and had come to the conclusion that they came from outside the Commission.