Last week the EU stepped up its crackdown on so-called government "golden shares" in former state-owned assets, with the Netherlands, Italy and Portugal all suffering reversals. A spokesperson for commissioner Charlie McCreevy said, "Golden shares do not have a place in the internal market."

A Department of Transport spokesman denied the government had ever used the disfavoured term "golden share" to describe the stake the State will retain in Aer Lingus following a flotation.

But a 2004 report by Goldman Sachs prepared for the government on the future of Aer Lingus specifically recommended that the government find a way to keep a "golden share" in the airline.

Transport minister Martin Cullen said that a minimum 25.1% stake in Aer Lingus would enable the government to protect its "strategic interests".

However, given the newly prevailing attitudes in Brussels, the definition of "strategic interests" the government will be able to defend seems far more limited than even a few months ago and there may be little that could be done to prevent a majority shareholder making sweeping changes.

A Department of Transport spokesman said the government will adhere to requirements and maintained that its minimum 25.1% stake would make it likely that it remains the single strongest shareholder.

Aer Lingus needs to raise 400m from the stock market to allow it to borrow the 1.6bn to fund the acquisition of additional long-haul aircraft. Siptu national industrial secretary Michael Halpenny said on Friday that the Aer Lingus sale process had not been properly thought through.

"They should rethink the issue, " he said. "The government is saying that from a strategic point of view the retained shareholding will be its weapon of choice." He added it is "not in the government's gift" to assure it can prevent changes at the airline. Siptu has already served strike notice on Aer Lingus.

Labour leader Pat Rabbitte, who raised the matter in the Dail last week, told the Sunday Tribune that recent EU rulings showed that the claim the government could use its 25.1% stake to protect strategic intrerests was "deceitful and misleading".

Ryanair chief executive Michael O'Leary told the Sunday Tribune that the government is "giving away a national asset" and said the State would not be able to guarantee the protection of Aer Lingus.