International Consolidated Airlines Group (IAG) completed the move, but in return for Brussels' agreement, BA's owners must give up pairs of daily take-off and landing slots at London Heathrow to boost competition in the sector.
Other commitments of the agreement include seven daily take-off and landing slot pairs to be used between Heathrow and either Edinburgh and/or Aberdeen, as well as five daily points between the London Airport and various destinations.
Willie Walsh, Chief Executive of IAG, said the acquisition of the airline can boost British employment opportunities.
However, pilots' union Balpa are concerned the deal could put jobs at risk in the future.
The Union's general secretary, Jim McAuslan, added: "IAG has so far made it clear that they only intend to run the bmi mainline operation and that although they must take over the bmibaby and bmi regional businesses as part of the transaction, these businesses do not figure in their plans."
The airline's Belfast-Heathrow route is secure following BA's acquisition of bmi and Northern Ireland's Secretary of State, Owen Paterson believes the move will boost economic growth.
"Heathrow gives opportunities for global connections to and from Northern Ireland that are not available at the other London airports. Those connections are absolutely vital to our investment and trade."