Budget airline Ryanair has now become "kingmaker" in the long-running IAG bid to buy Aer Lingus.
IAG chief executive Willie Walsh said he expects the airline to act in a "rational way" to its takeover bid for the Irish flag carrier.
Michael O'Leary's Ryanair owns almost a 30% stake in Aer Lingus, and after the Irish government supported the takeover, the decision on whether to plough ahead with the deal rests on his shoulders.
But it has been keeping its cards close to its chest, saying it had yet to receive an offer despite the announcement that terms had been agreed by the Aer Lingus board.
And signalling his confidence that Ryanair would now accept the IAG bid, Mr Walsh said that there was a "compelling offer" on the table.
"I believe Ryanair will see the merit of the case we have made, the value we are offering in terms of this takeover and will want to see the deal go through," he said.
Mr Walsh said he has not talked to Ryanair in the last few weeks and could not cut any deal with the low-cost carrier on routes or other arrangements, under competition and takeover laws.
IAG is offering Aer Lingus shareholders €2.55 (£1.81) in cash per share, under the proposed deal backed by the airliner's management. However it's been disputed by both trade unions and opposition parties.
Mr Walsh said that offer "was the limit".