UPDATE 2-UK signs memo with US on Joint Strike Fighter

(Recasts; adds Drayson quotes, Australia signing, details; previously datelined LONDON)

WASHINGTON, Dec 12 (Reuters) - Britain signed a framework agreement with the United States on Tuesday outlining conditions to buy and operate the Joint Strike Fighter, but stopped short of committing to buy the combat jets.

Britain had been concerned that tight U.S. controls on sensitive defense technology exports might leave it reliant on the United States to operate and upgrade the aircraft for which Lockheed Martin Corp. LMT.N is the prime contractor.

“We have today received the necessary assurances from the U.S. on technology transfer, which we would require to operate the aircraft safely and maintain, repair and upgrade it,” the Ministry of Defence said in a statement on Tuesday.

While Britain has not formally committed to buying the aircraft, Britain’s Minister for Defence Procurement, Lord Drayson, said this was the intention.

“Our Plan A is to buy the Joint Strike Fighter,” Drayson told reporters in Washington, where he signed a memorandum of understanding with U.S. Deputy Defense Secretary Gordon England.

"However, it is absolutely right for us to have a Plan B." Drayson added. He did not elaborate, but alternatives for the UK could include development of a carrier-based version of the Eurofighter Typhoon or purchase of the Rafale built by France's Dassault Aviation AVMD.PA.

Recently renamed the Lightning II, the radar-evading F-35 Joint Strike Fighter is being developed by the United States, UK and seven other nations to replace F-16s, F/A-18 Hornets and other aging combat aircraft.

Australia also advanced its participation in the F-35 project on Tuesday, while Canada signed an agreement outlining its role in the program on Monday, and the Netherlands signed last month.

Italy, Turkey, Denmark and Norway are the other partners in the aircraft that may have its first flight this week.

At a projected $276.5 billion, the warplane represents the Pentagon’s costliest planned purchase, with more than 2,400 aircraft planned by 2027 for the U.S. Air Force, Navy and Marine Corps.

Britain aims to use the planes aboard two aircraft carriers it is planning to build for the Royal Navy. Britain's BAE Systems BA.L is involved in development of the plane.

Britain had been pushing for the Pentagon to ease technology transfer rules and grant access to source codes needed to upgrade the plane’s computer software.

Asked if source codes were part of the technology transfers, Drayson said they were.

Britain also received clarity that there would be an unbroken British chain of command operating its aircraft. Britain would “not be required to have a U.S. citizen in our own operational chain of command,” Drayson said.

Drayson said he understood the project was going forward with development of two engines for the F-35. Britain was angered earlier this year by Pentagon plans to eliminate a second engine to be built by Britain's Rolls Royce Group Plc and General Electric Co. GE.N. But Congress restored funding for the second engine.