Japan has chosen the US-made F-35 stealth jet as its next-generation mainstay fighter in a multi-billion dollar deal, reports said Tuesday.

The defence ministry picked the jet made by Lockheed Martin to replace its ageing fleet of F-4 jets over two rivals, the Boeing-made F/A-18 Super Hornet and the Eurofighter Typhoon, the Yomiuri Shimbun said.

Japan's biggest daily said the defence ministry had "agreed in principle" to select the F-35, with a formal announcement expected Friday at the Security Council of Japan, chaired by Prime Minister Yoshihiko Noda.

Kyodo News cited unnamed government sources as saying the ministry had decided to buy 40 F-35 fighters, while the Nikkei business daily said only that the jet remained a "prime candidate".

Earlier reports said Japan could buy as many as 50 new jets, equipped with stealth technology, with a price tag of more than $6 billion.

A defence ministry spokesman on Tuesday declined to confirm the reports, saying: "We are still trying to do our best to make a final decision by the end of this year."

And Chief Cabinet Secretary Osamu Fujimura denied the reports that a final decision had been made, Dow Jones Newswires said.

The F-35, the most expensive weapons programme in Pentagon history, has been plagued by cost overruns and technical delays.

The jet, co-developed with British defence giant BAE Systems, is the most expensive among the three candidates, with a price tag estimated at $113 million per aircraft.

Japan initially aimed to acquire the F-22 stealth fighter to renew its fleet, but US law prohibits exports of the jet with the United States having said it would halt production of the model.