Adding extra conditions to the sale of Aer Lingus to IAG - such as forcing it to release its prized slots at Heathrow - may not be enough to avoid a monopoly on the route.
That's according to Virgin Atlantic, which says no other airlines will be able to operate on the Dublin-Heathrow route - even if extra slots become available.
If the long-mooted takeover bid were to go ahead, IAG would run all 19 daily flights from Dublin to Heathrow.
Currently Aer Lingus and British Airways are the only airlines to fly on the route, which is one of the busiest international routes on Earth.
IAG would expect to be forced to hand over some of those routes to rival airlines if the takeover was to go ahead.
But Virgin Atlantic says even if this were to go ahead, other airlines could find it almost impossible to take up those slots and operate viably on the route.
Its head of Networks and Alliances, Joe Thompson, told the Oireachtas Transport Committee today that Virgin has experience of similar problems in recent years.
He told the committee that Virgin had take over some Heathrow slots which were made available as part of IAG's purchase of BMI (formerly British Midland) in 2012.
Those slots were used to compete with IAG's airlines on routes between Heathrow and the Scottish cities of Aberdeen and Edinburgh.
But, he said, it proved impossible to run a viable service on those routes, as all three airports were dominated by IAG airlines - and eventually those routes were abandoned.
Mr Thompson also told TDs that Virgin Atlantic would expect to lose its commercial partnership with Aer Lingus if it was taken over by IAG.
Currently Irish passengers hoping to travel to the United States or elsewhere can buy Aer Lingus and Virgin Atlantic tickets at the same time, allowing them to make a single booking to the UK for an Aer Lingus flight, before taking a Virgin flight to one of its worldwide destinations.
This would likely disappear if Aer Lingus was sold, as Aer Lingus would instead be likely to contribute passengers to British Airways' long-haul flights from London Heathrow instead.
This would mean fewer options for long-haul international travel for Irish passengers, Mr Thompson said.
