British Airways and two other airlines have received approval to operate a joint business on transatlantic flights.
Sir Richard Branson, president of Virgin Atlantic, said millions of passengers on both sides of the Atlantic would suffer the consequences of the "monster monopoly" and that it was "a bad day for consumers".
BA, American Airlines (AA) and Iberia have been granted anti-trust immunity from the US Department of Transportation, BA said.
BA chief executive Willie Walsh said: "This final approval is fantastic news for all three airlines and the oneworld alliance.
"We've waited 14 years to bring the benefits of the transatlantic joint business to our customers and level the playing field with the other two global alliances.
"As we have argued all along, the EU-US market is highly competitive and Heathrow's liberalisation in 2008 opened it up even further. We are delighted that the US and EU authorities have recognised this."
American Airlines chief executive Gerard Arpey said: "This is a great day for the customers, employees and shareholders of American Airlines and our joint business partners in the oneworld alliance. "We thank all of the regulators who reviewed our proposals, both here in the United States and in Europe, and all those persons and organisations that supported us in this effort."
Iberia's executive chairman Antonio Vazquez said: "We're delighted we have received approval for our joint business as it will be very positive for our employees, our shareholders and, most important of all, our customers."
The airlines plan to launch the transatlantic joint business this autumn and will co-operate commercially on flights between the EU, Switzerland and Norway and the US, Canada and Mexico.
Sir Richard said: "It is clear that in taking this decision, the US DOT has decided to put the interests of BA and AA before those of the flying public. As a result, millions of passengers on both sides of the Atlantic will suffer the consequences of this monster monopoly. This is a bad day for consumers."