British Airways is one of the three airlines involved
The European Commission's competition authorities have raised concerns about the transatlantic plans of British Airways, American Airlines and Iberia.
The Oneworld alliance members want to operate as a joint business on flights between the EU and North America.
If they are to go ahead they will also need approval from US regulators.
The Commission has sent its Statement of Objections to the airlines, and they now have the chance to respond before a competition decision is reached.
The Commission confirmed that it was still looking into similar plans for co-operation by Star Alliance members Lufthansa, Continental, United and Air Canada and between Skyteam members Air France/KLM and Delta/Northwest.
The Oneworld team's plans would allow them to co-operate on pricing and schedules for transatlantic flights.
It would also mean that if customers contacted one of them to buy tickets on such flights, they would be offered flights provided by all three.
In a statement, BA said it was, "looking forward to the opportunity to address and overcome the EU's concerns".
"Our joint business would benefit millions of customers with greater access to discounted fares, more convenient connections and better access to a global network of more than 500 destinations," BA said.
BA already operates two joint businesses - with Qantas on flights to Australia and with Iberia on flights to Spain - neither of which needed EU regulatory approval.
The plans of Star Alliance and Skyteam have already received approval from the US Department of Transportation.
BA expects the US regulators to rule on the Oneworld plans by the end of the month.