Aer Lingus has said a revised takeover bid by rival airline Ryanair undervalues the business.
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.
Ryanair offered €1.30 (£1.02) a share and the Irish government, which retains a stake in the airline since privatisation, said it will look at the new takeover attempt.
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 per share fundamentally undervalues Aer Lingus."