And, Mr Tusa said, it would leave the UK without a plane to put on its new Queen Elizabeth-class aircraft carriers: "This is the only plane with vertical landing".

Liz Quintana, air power expert at the Royal United Services Institute agreed that if America decided to cancel the programme, it would leave Britain and other European allies who have placed orders "a bit stuck".

She said there was no alternative so-called fifth generation stealth fighter available anywhere else, or even on the drawing board.

She said: "The US Air Force would be as stuck as anyone else in terms of what it buys instead."

If America cut its order numbers, it would also drive up the overall cost of UK orders.

Britain has agreed to buy 138 of the fighters over the next two decades and to buy 24 by 2023.