The Japanese government has delayed a formal announcement on its choice of a next-generation fighter jet until December 20, but Lockheed Martin Corp's radar-evading F-35 is still expected to get the order, one source familiar with the process said on Wednesday.

The delay came because Japan's national security council, chaired by Prime Minister Yoshihiko Noda, has to deal with other pressing matters at a December 14 meeting where the fighter jet decision had initially been expected, said the source, who was not authorized to speak on the record.

Japan's government and ruling party officials have approved a Defence Ministry proposal to buy Lockheed's F-35 Joint Strike Fighter as the country's next mainstay fighter, public broadcaster NHK said on Wednesday.

Japanese media said on Tuesday that Lockheed was likely to win a deal worth over $7 billion (4 billion pounds), beating out Boeing's F/A-18 and the Eurofighter Typhoon, made by a consortium of European companies including BAE Systems.

Japan's Defence Ministry denied the reports, and the U.S. Defence Department office that oversees the F-35 program said it had not been notified by the Japanese government of any decision.

The Defence Ministry has made no decision yet. A meeting of minister, vice minister and parliamentary secretaries (to make the decision) has not even been held yet, a spokesman for the ministry said.

Japan, which counts the United States as its key security ally and regularly conducts military drills with U.S. forces, had been widely expected to choose the F-35 because of its advanced stealth capability and U.S. origin.

The aircraft's stealth technology has drawn much attention in Japan since China, which has a long-running territorial dispute with Japan, in January confirmed it had tested for the first time its J-20 stealth fighter jet.

(Reporting by Kiyoshi Takenaka and Andrea Shalal-Esa, editing by Dave Zimmerman)