A proposed merger between British Airways and American Airlines would be like "allowing Coca Cola and Pepsi to merge", Sir Richard Branson said.
The Virgin Atlantic boss said the link up between the two giant carriers would have a hugely damaging impact for the travelling public.
The union would give BA and AA a virtual stranglehold over travel agents and corporate customers he said.
The proposed deal would involve 443 destinations in 106 countries and 6,200 departures in total.
Virgin said that BA and AA had failed to give regulators and the public a true picture of their market share between London Heathrow and the US, by leaving out bookings made direct with the airlines and passengers making connecting journeys.
Virgin said that BA and AA's data which said they had a 40% share of passengers across the Atlantic, was around 20% below the true figure.
Sir Richard promised to mount a strong campaign against the merger, unveiling a slogan to be painted on all Virgin Atlantic planes, which read: "No way BA/AA".
He said that it was vital the competition regulators maintained a level playing field at what is a very difficult time for airlines.
"The regulator's job, even in times as difficult as these, is to regulate," he said. "What BA and AA want to do is effectively merge and that will reap unbelievable damage. It's like allowing Coca Cola and Pepsi to merge. It's unthinkable that a competition authority could allow such a merger to happen."
Virgin is planning to stage a major advertising campaign to highlight its belief that the planned alliance would reduce competition and push up prices.
