EU officials have raised competition concerns over Heathrow landing rights and transatlantic routes after IAG outlined its formal offer to buy Aer Lingus last week.
The EU's competition law directorate raised concerns with IAG over some aspects of its €1.4 billion bid for Aer Lingus at a meeting attended by both companies in Brussels in the last few days.
The mergers watchdog is checking whether an IAG takeover of Aer Lingus would be bad for competition in landing and take-off slots at London's Heathrow Airport.
IAG had agreed with the Government that Aer Lingus would retain ownership of its slots and that they would continue to be used to serve routes between the Republic and Heathrow for seven years.
This element of the deal was central to the Government agreeing to sell its 25.1% stake in the airline to IAG.