Aer Lingus has asked its shareholders to reject a takeover offer from Ryanair, calling it "ill-conceived, contradictory and anti-competitive".
Aer Lingus says its passengers prefer it to Ryanair
In a circular to investors outlining its many reasons for opposing a tie-up, it said the Ryanair bid "significantly undervalues Aer Lingus".
Ryanair made its £1bn bid last month, telling Aer Lingus staff they could earn £40,000 selling their shares.
The Irish government has a 28% stake in Aer Lingus and has refused to sell it.
'Small, regional airline'
"Over the past five years, Aer Lingus has been substantially transformed," said the airline's chairman, John Sharman.
"Faced with a strong, well-capitalised and independent Aer Lingus, Ryanair, our principal short-haul competitor, has reacted in a hostile, anti-competitive manner designed to eliminate a rival at a derisory price."
Mr Sharman also took a swipe at the two airlines' different approaches to business.
"Our product offering is differentiated from our peers. It is superior to Ryanair's and passengers prefer to fly with Aer Lingus."
In its official offer document, issued last month, Ryanair said Aer Lingus would remain a "small, regional airline" if its shareholders did not accept the offer.
It also said that Aer Lingus would be "at the mercy" of its government and workforce shareholders, who had exercised "de facto control" over the airline in recent years.