Aer Lingus opposes Ryanair offer
In a letter to shareholders urging them to reject the offer, the carrier said the reasons were stronger than at the time of the first failed bid in 2006.
Aer Lingus said a takeover would mean the number of routes that Ryanair monopolises would sharply increase.
The airline also said it has legal advice that the European Commission is likely to again ban the takeover.
An investigation is also being carried out into Ryanair's current 29% holding by the UK Competition Commission.
"Aer Lingus is a robust and profitable airline with a proven business model, a strong balance sheet and an internationally recognised brand," the airline said.
"Your board's unanimous view is that Ryanair's offer to acquire control of Aer Lingus for 1.30 euro per share fundamentally undervalues Aer Lingus."
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