The CEO of Aer Lingus Christoph Mueller has said Ryanair's bid for the company is too low, and would be bad for consumers.
The board of Aer Lingus has urged its shareholders to reject Ryanair's offer of €1.30 per share, which translates into a company valuation of €694m.
Aer Lingus said the offer "fundamentally undervalues" Aer Lingus.
Mr Mueller also the company's legal advisors did not think the European Commission would allow the sale to go through.
"We have advised our shareholders that this offer is not in their interests," he said.
"It is incapable of completion - that is the legal advice we received with regard to the competition commission - and the price offer undervalues the company significantly."