The AFP confirmed to Fairfax Media on Friday the investigation into the leak - the second in two months on the bid - had started.

In March, an investigation began after an apparent leak to The Australian of sections of the draft Defence white paper. This time, the investigation is looking at a leak to the same newspaper that suggested Japan - which along with Germany and France is bidding to build the subs - was considered by cabinet's National Security Committee to have the weakest bid.

Fairfax Media has been told that the Secretary of the Defence Department, Dennis Richardson, was outraged by the latest leak and personally requested the investigation, but Defence declined to comment by deadline.

In a statement, an AFP spokeswoman said: "The AFP can confirm it has received a referral from the Department of Defence regarding the possible unauthorised release of government information. As the matter is ongoing it would not be appropriate to comment further."

Australian Defence Association executive director Neil James said that "based on previous leaks of this nature over many years, it is more likely to have come from the Parliament than the department or the Australian Defence Force".

The investigation comes as Japanese Prime Minister Shinzo Abe is considering a direct call to Malcolm Turnbull in a last-ditch attempt to revive his country's bid to build Australia's new submarines.

Amid heightened speculation that the Australian government is about to announce within days the winner of the contract, suggestions Japan has already been ruled out have left the Asian nation alarmed.

The government would not say if a decision had already been made and would be announced next week, but reports in Australia and internationally have claimed both France and the Germany as the frontrunner.

The matter has been before cabinet twice. The National Security Committee, and then the full cabinet, is expected to meet again next week for a final decision. Prime Minister Turnbull would then announce the successful bidder immediately after.

Neither German shipbuilder TKMS nor France's DCNS have been informed of the decision.

Japan's Mitsubishi Heavy Industries is thought to have lagged behind the Europeans because of technical and engineering concerns. But sources say the Japanese are not giving up and that the competitive evaluation process has Tokyo's attention at the highest levels.

A spokesman for the Japanese embassy in Canberra said they had not been informed of any decision yet. "We would like to know what is happening now, too," he said.

Both the embassy and Mr Turnbull's office confirmed that Mr Abe had not yet called his Australian counterpart in an effort to rescue the bid. But other Japanese sources insist such a call is being considered.

"Those engineering issues can easily be addressed and the government of Japan would like the government of Australia to know that," one said.

Should Mr Abe make the call, it is believed he will remind Mr Turnbull that his predecessor, Tony Abbott, had recommended the Japanese.

"Comments like that are viewed very strongly in Japan," the source said. "It's a matter of honour."

But another contact said the Japanese were "beside themselves with angst" that they would lose this bid. "It will be a disaster for them, because they will lose face."

Industry sources close to the tendering process said the French might have the better submarines but the contract was  there for the Germans to lose.

"The government has done very well at keeping its cards close," one said. "No one knows the decision yet and Turnbull will want to control the message. He won't even tell cabinet until it is final and he is about to announce it. He is scared of cabinet leaks."

Another contact suggested that announcing either the Germans or Japanese as the successful bidders would not be a "good look" over the Anzac Day period.

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