BRUSSELS (Bloomberg) — European Union regulators started an in-depth investigation into Ryanair Holdings Plc’s plans to buy Aer Lingus Plc on concerns the takeover will restrict competition for flights from Dublin to London.The EU will review the 1.48 billion-euro ($1.95 billion) hostile bid after Ryanair’s offer to sell slots at eight airports, including London’s Heathrow, failed to overcome objections. Regulators will rule on the transaction by May 11, 2007.

The proposed deal would “raise serious concerns” as it could “reduce choice for consumer and could give rise to higher fares than would be likely if the two carriers remained separate,” the European Commission, the 25-nation EU’s antitrust regulator in Brussels, said in a statement today.

The offer, which expires tomorrow, is also facing shareholder resistance. It has been rejected by the Irish government and Aer Lingus employee shareholders, who together control almost 40 percent of the carrier. Ryanair, which holds a 25.2 percent stake, has said it won’t raise the offer.

Peter Sherrard, a spokesman at Ryanair, had no immediate comment.

The commission’s concerns about the bid are “in line with our stated view,” Aer Lingus said in an e-mailed statement. The carrier said it will cooperate with the EU investigation.

Michael O’Leary, chief executive of Europe’s biggest discount airline, has pursued the takeover of Ireland’s flag carrier to add routes to the US Ryanair last month told the EU it would sell as many as four Aer Lingus slots at Heathrow if its bid succeeds, according to a document submitted to the commission. It also offered slots at Stansted Airport near London and airports serving Frankfurt, Paris, Milan, Rome, Bratislava and Barcelona.