Aer Lingus has recommended its shareholders reject Ryanair's takeover bid, claiming the offer of €1.30 per share "fundamentally undervalues" the flag carrier.
In a statement, Aer Lingus describes Ryanair's offer as not "credible", not least that the European Commission is likely to block the deal.
It says: "Ryanair's 2006 offer was prohibited by the European Commission on competition grounds, and [we] believe that the reasons for prohibition are now even stronger than before: the number of routes that Ryanair would monopolise has sharply increased.
"[We have] received legal advice that the European Commission is likely once more to prohibit the Ryanair offer, and that this is not therefore a credible offer which is capable of completion."
A UK Competition Commission probe into the anti-competitive effects of Ryanair's 29.82% stake in Aer Lingus is also likely to go against the low-cost carrier, it claims, with the likely result that Ryanair will be forced to sell its holding.
It adds: "Aer Lingus' strategy of building a leaner and more efficient business is working. Operational and financial performance has improved greatly since 2009, resulting in a turnaround in operating result since that time of approximately €130 million. We have transformed a loss-making Aer Lingus into a profitable airline with one of the strongest balance sheets in the European sector."