The union representing Aer Lingus pilots has said the airline was "mischief-making" in leasing Ryanair planes to cover services during the planned two-day pilots' strike this week.
Aer Lingus, meanwhile, has denied suggestions that the hiring of its rival's planes is a provocative move.
The IMPACT union said Aer Lingus should be trying to avoid the strike by entering into talks over pay and conditions for pilots at the new Belfast hub.