British Airways (BA) and American Airlines (AA) have been given initial permission by the United States Department of Transport (US DoT) to cooperate on schedules and ticket pricing.
If and when it is ultimately granted, the anti-trust immunity (ATI) would make all carriers in the Oneworld alliance - including Iberia, Finnair and Royal Jordanian - immune from competition laws and allow them to compete on a level playing field with rival alliances Skyteam and Star Alliance.
The news was a blow for BA's rival Virgin Atlantic, which has waged a long-term campaign against the tie-up, claiming it would push up airfares for consumers and give BA a competitive advantage at Heathrow where it already has some 40% of take-off and landing slots.
Sir Richard Branson was known to have been furious at the decision, saying US government officials should be "ashamed of themselves".
Virgin has only 45 days to make its final objections as a concerned party before a final decision is made.
In return for ATI the US DoT has demanded that BA and AA between them give up four slots at Heathrow - but it is understood the airlines would only have to lease them, rather than sell part of what is arguably their most valuable asset.
With Heathrow already running at full capacity neither BA nor AA should have difficulty finding a buyer or tenant.
BA's chief executive Willie Walsh was understood to happy with the news. The Irishman had previously said the ability to coordinate fares with fellow Oneworld carriers would be beneficial to the consumer and would give the companies parity with Star Alliance and Skyteam who already command competition immunity.
BA said it would operate a joint business with AA and Iberia between Europe and North America.
A spokesman said: "As verified by the US DoT, closer cooperation, made possible by anti-trust immunity, will benefit customers with more travel choices and convenient schedules, expanded opportunities to earn and redeem frequent flyer miles, and greater availability of lower fares."
Virgin will continue to lobby the European Union where politicians and bureaucrats are continuing to review the case.
But the news was a second boost for AA in less than a week after Japan Airlines pledged to remain as its key partner in the Far East.
JAL had been courted by rival alliances, but last week it committed to the AA partnership as they applied for separate ATI.