Green Party Transport spokesperson Eamon Ryan has claimed that Ryanair may already own a 25 per cent share in Aer Lingus.
If true, that could extend certain legal rights over the future direction of the former State airline to Ryanair.
The claim comes following the revelation that a legal challenge may be mounted by the Government to Ryanair's bid to take over the carrier.
In a shock move yesterday, Ryanair tabled a bid for €1.4 billion for Aer Lingus. Ryanair said the offer was conditional on obtaining at least a majority of the shares in Aer Lingus.
Mr Ryan said: "Ryanair may already have acquired a blocking control of Aer Lingus by this lunchtime. Over 16 million shares in the company have already been traded today which accounts for almost 8 per cent of the companies issued share capital.
"If Ryanair have purchased even two-thirds of the shares that have been traded then they will have increased their shareholding above 25 per cent, which will give them effective blocking control over the development of the company," he said.
Ryanair has offered shareholders €2.80 a share, a 27 per cent premium on the opening price of €2.20 when Aer Lingus was listed on the stock exchange last Monday.
"Anyone who owns more than 25 per cent of a company has the right to defend their interests in the courts as a significant minority shareholder. While this would not give Ryanair control of the day-to-day running of the company, it would allow it to block any major capital expenditure which it could argue was not in its own interest," Mr Ryan claimed.
It is understood the Government is considering a legal challenge on competition grounds.
Chief executive Michael O'Leary said the Ryanair takeover would give the Government €500 million for its shares and would give Aer Lingus employees €60,000 each.
But the Employee Share Ownership Trust, which represents the staff shareholding, argued last night that when tax was taken into account the actual figure was likely to be much lower.
As of last night, Ryanair held almost 20 per cent of shares in Aer Lingus, up from the 16 per cent it held yesterday morning. The airline's share price is trading up 7 per cent at €2.97 at 3.45pm.
Ryanair shares are also up at €8.77.
The Government held on to a 28.3 per cent stake in Aer Lingus when the airline was floated last Monday.