British entreprenuer Richard Branson, founder of Virgin Atlantic, has claimed it would be "absolutely wrong" for IAG, the owner of British Airways, to buy Aer Lingus.
He wants competition authorities to move to block the takeover, he said in an interview with Britain's Telegraph newspaper.
The billionaire said the takeover should "not be allowed", and that the Irish airline should continue as an "independent competitor" to BA.
"I think it's absolutely wrong," Richard Branson said.
"I find it incredible that the competition authorities allow such things to happen."
However, the Virgin boss admitted his own airline looked at mounting its own bid for Aer Lingus.
Virgin Atlantic and British Airways have been fierce rivals on the transatlanic market for years.
The possibility of handing Aer Lingus' crucial Heathrow airport landing slots to IAG is one of the British entreprenuer's main concerns.
His Virgin Group owns 51pc of Virgin Atlantic.
Virgin secured an increased share of Heathrow landing slots after IAG bought bmi, when regulators insisted that the combined IAG holding of the sought-after landing slots was too high.
Virgin then partnered with Aer Lingus to create the Little Red brand to operate some of those slots for short-haul operations but that venture was scrapped last year in part because it struggled against competition from BA.
Any increase in IAG's access to Heathrow from buying Aer Lingus should also be examined, according to Richard Branson.
"The competition authorities should look very closely at the Aer Lingus deal. It's just giving BA more power, more slots," he said.
"We will argue very strongly that British Airways have already got a complete dominance of slots at Heathrow and that those should be allocated to other companies," he said.
"They've got far too many slots anyway. If it was any other industry they would have been broken up by now."
An IAG tie-up with Aer Lingus would also inhibit competition into Ireland, he said.
"You're taking away valuable competition on the Irish routes by allowing BA to take them over," he claimed.