The European Union’s antitrust regulator said on Friday that it had sent objections to British Airways, American Airlines and Spanish flag carrier Iberia over their proposed joint venture, amid complaints from rival airlines that it would stifle competition on routes between London Heathrow Airport and several cities in the US.

British Airways, American and Iberia — already members of the global Oneworld airline alliance — have proposed a deepening of their existing partnership that would allow them to share revenue and make joint marketing and scheduling decisions on flights between the US, Mexico, Canada, and the 27-member EU, as well as Switzerland and Norway. The proposed agreement would create a unified network of around 500 possible destinations in more than 100 countries.

Lawyers for the three airlines will spend the next few weeks studying the European list of objections, which are understood to center around the dominant market position that a deal would give British Airways and American Airlines, in particular, on links between Heathrow and destinations like Dallas-Fort Worth, Boston and Miami.