Britain could withdraw from a deal to buy the Anglo-American F-35 Joint Strike Fighter unless the US shares military secrets, a minister has said.
US defence firm Lockheed Martin is leading the project
Defence procurement minister Lord Drayson made his position clear in an address to the US Senate's Armed Services Committee in Washington.
Last year the UK told the US it must provide the "technology transfer" needed to maintain and upgrade the JSF.
Britain committed $2bn (£1.15bn) to developing the stealth fighter in 2001.
It also became the first of eight countries to join the F-35 JSF project.
In December, the Commons Defence Select Committee said the UK needed "cast-iron assurances" that the US technology transfer would take place.
Lord Drayson was in Washington to discuss ongoing disputes over the aircraft.
He said: "Without the technology transfer to give us aircraft that are fit to fight on our terms, we will not be able to buy this aircraft."
In August, Rolls-Royce won a £1.3bn ($2.47bn) deal to develop a second engine for the JSF, which is intended to replace the current generation of fighters.
Lord Drayson said plans by the Bush administration to scrap this second engine could threaten British involvement.
He told the committee in Washington that Britain expected to be consulted before cuts to the engine programme were proposed.
But a spokesman for the Ministry of Defence played down the threat of a split with the US.
"Lord Drayson is clear he can find a way through that will meet our requirements for sovereign capability," he said.