WASHINGTON - In a move that has sparked bitterness among supposed collaborators, the United States is to keep to itself sensitive software code that controls Lockheed Martin's new radar-evading F-35 fighter despite requests from partner countries.

Access to the technology had been publicly sought by key ally Britain, which had threatened to scrub plans to buy as many as 138 F-35s if it was unable to maintain and upgrade its fleet without US involvement.

No other nation will get the so-called source code, the key to the plane's electronic brains, Mr Jon Schreiber, the Pentagon official heading the programme's international affairs, said. He admitted this had displeased the eight nations co-financing F-35 development: Britain, Italy, the Netherlands, Turkey, Canada, Australia, Denmark and Norway.

The single-engine F-35, also known as the Joint Strike Fighter, is in the early production stages. It is designed to escape radar detection and switch between air-to-ground and air-to-air missions while aloft - tricks heavily dependent on its eight million lines of onboard software code.

Mr Schreiber said the US had accommodated all of its partners' requirements, providing ways for them to upgrade projected F-35 purchases even without the keys to the software.

"Nobody's happy with it completely. but everybody's satisfied and understands," he claimed. It is also a rebuff to Israel, which has sought technology transfer as part of an F-35 purchase.

Instead, the US is to set up a "reprogramming facility", probably at Florida's Eglin Air Force Base, to further develop F-35-related software and distribute upgrades. Software changes will be integrated "and new operational flight programmes... disseminated to everybody who's flying the jet," said Mr Schreiber.

Britain had threatened to bail out in 2006 if the US withheld such things as the software code. In May 2006, then President George W. Bush and then Premier Tony Blair announced "the UK will have the ability to successfully operate, upgrade, employ, and maintain the Joint Strike Fighter such that the UK retains operational sovereignty over the aircraft".