Aer Lingus has issued a letter to its shareholders urging them to reject Ryanair's bid for the business.
The airline has offered €694m, or €1.30 per share, for Aer Lingus.
The letter, from the chairman of the board of Aer Lingus Colm Barrington, said that the offer undervalues the airline and is likely to be rejected again by the European Commission on competition grounds.
"Ryanair indicated its intention to bid for the Group in June and launched a formal offer for Aer Lingus on July 17, 2012," said Aer Lingus CEO Christoph Mueller..
"The board of Aer Lingus has unanimously recommended that shareholders reject Ryanair’s offer by taking no action."