Adding extra conditions to the sale of Aer Lingus to IAG may not be enough to avoid a monopoly on the route, Virgin Atlantic has said.
The airline said no competitors would be able to operate on the Dublin-Heathrow route even if extra slots became available.
IAG would run all 19 daily flights from Dublin to Heathrow if Aer Lingus is sold.
Virgin's Joe Thompson said it has similar experience from when IAG bought the airline BMI in 2012.
"A competitive remedy was applied which made available short-haul slots for airlines to compete on routes that became a monopoly as a consequence (of the BMI buy-out). Virgin Atlantic started operating between Heathrow and Aberdeen and Edinburgh using these remedy slots," he said.
"Unfortunately, competing with a monopoly provider is a very difficult thing to do and we could not make those services economic."