Virgin Atlantic Airways has warned the European Commission the proposed sale of Aer Lingus to the International Airlines Group (IAG) could have serious anti-competitive effects.
In a submission seen by RTÉ News, it says that should the sale go through "unchecked" it could lead to higher prices and a deteriorating service for consumers.
It told the Commission the merger would create a monopoly on some routes.
These include London Heathrow to Dublin, Belfast to Heathrow and Dublin to Chicago.
The Richard Branson-founded airline says that without safeguards the passengers who fly on these routes will be adversely affected by the transaction.
The submission states that every year over 700,000 passengers fly from Dublin to connect to long haul flights at either Heathrow, Gatwick or Manchester.
Virgin claims IAG will have the ability to divert these Aer Lingus passengers away from other long haul carriers and the incentive to funnel them to IAG.
Virgin Atlantic wants the European Commission to only approve the sale if it provides "effective remedies" to deal with what it says are the "detrimental effects on competition."