British Airways, American Airlines and Iberia may have to give up take-off and landing slots for their Oneworld transatlantic alliance to go ahead, the Financial Times reported on Monday.
The airlines had planned to deepen the pact to take advantage of the US/EU "Open Skies" agreement, with the focus on routes between the United States, Mexico, Canada, the EU, Norway and Switzerland.
But earlier this month European Union regulators said the latest plan may violate antitrust rules on restrictive business practices.
The Financial Times, citing documents seen by the newspaper, said on Monday the European Commission found that the proposed tie-up was likely to result in "appreciable competitive harm" on seven European-US routes.
Remedies may include the transfer of slots to other carriers, the newspaper said citing comments made by the European Commission.
Alliances are seen as a lucrative alternative to mergers and large-scale investments. The three carriers have applied for US government antitrust immunity for their trans-Atlantic pact.