The US Transportation Department has conditionally approved an alliance between American Airlines and British Airways.
The companies want to work together to set rates and routes and sell each other's tickets.
The airlines must give away 224 takeoff and landing slots for travel between US cities and Heathrow Airport, before they can get antitrust immunity for their alliance.
Transportation Department officials say the agreement would allow four new US airlines to fly to and from Heathrow.
American Airlines and British Airways have been seeking federal permission to form an alliance since 1996.
The department says final approval of the partnership and another between British Midland and United Airlines was dependent on reaching a new, more open, aviation trade pact with the UK.
PA