Sir Richard Branson warned on Thursday that British Airways’ planned transcontinental venture with American Airlines could imperil Virgin Atlantic Airways, the carrier he founded 25 years ago, if it wins approval from US regulators.

In his most searing critique yet of his rivals’ proposal, Sir Richard said the venture would grant these rivals a dominant share of key take-off and landing slots at London’s Heathrow Airport, the world’s most important international gateway, weakening their competitors and leading to higher fares and fewer transatlantic flights.