Japan will agree this month to give Australia classified submarine data, an unprecedented step signaling Tokyo's intent to join competitive bidding to sell Canberra a fleet of stealth subs, said two Japanese officials familiar with the plan.

The "competitive assessment" will see Germany's ThyssenKrupp and France's state-controlled naval contractor DCNS separately competing with a Japanese government-led bid for such contractors as Kawasaki Heavy Industries Ltd. and Mitsubishi Heavy Industries Ltd.

Japan had been the front-runner in the planned sale of around 12 vessels, for as much as $40 billion, to replace Australia's ageing Collins class submarines, sources have said, until Prime Minister Tony Abbott opened up the bidding under pressure from opposition and ruling party lawmakers.