TDs have been warned that it would be difficult for other airlines to run routes between Dublin and Heathrow, if Aer Lingus is sold to IAG.
The warning comes from Virgin Atlantic, which is giving evidence at the Oireachtas Transport Committee today.
If the takeover goes ahead, IAG would operate all 19 of the daily flights between Dublin and Heathrow.
Virgin's Joe Thompson says there have been problems before, when IAG took over the BMI airline which had a big presence in Scotland.
"Competing with a monopoly provider is a very difficult thing to do and we could not make those services economic," he said.
"So I think we've learned from that experience and the prospect of flying short-haul services, and in the case of the post Aer Lingus/IAG transaction, between two hubs that both belong to the competing airline, is not a very attractive commercial prospect."