Virgin Atlantic Airways
The European Commission agreed the takeover last month, saying IAG's concessions, which included a pledge to give up landing slots at London's Heathrow airport, Europe's busiest, were sufficient to address competition concerns.
But Virgin boss Richard Branson challenged this in a statement on Sunday, saying the deal would limit consumer choice.
He added that Virgin would bid for the slots being put up for sale.
(Reporting by Tom Bergin; Editing by Helen Massy-Beresford)