Prime Minister Shinzo Abe may be forced to delay his bid to reinterpret the Constitution to allow Japan to go to the aid of allies under attack, due to growing opposition within his ruling Liberal Democratic Party (LDP).
Failure to make the change during the current session of Parliament could upset plans to incorporate the new policy in US-Japan security cooperation guidelines due to be revised by the year end.
For more than 30 years, Japan has recognised that the country has the right to collective self-defence but chose not to exercise it in the spirit of the war-renouncing Article 9 of its Peace Constitution. What Mr Abe wants to do will, therefore, constitute a major policy shift.
LDP lawmakers wary of Mr Abe's national security ambitions want the policy change to be made via a constitutional amendment rather than a change of interpretation at the Prime Minister's discretion. The calls came during a two-hour meeting on Monday of the LDP's executive council.
"Changing the interpretation leaves a stain on the Constitution. We should openly discuss revision of the Constitution," said former reform minister Seiichiro Murakami. "If Bills are tabled in Parliament based on constitutional re- interpretation, I will have to oppose them," he added.
Retired lawmaker and former LDP secretary-general Makoto Koga said at a public lecture on the subject on Monday: "Changing a constitutional interpretation through a Cabinet decision is a patchwork measure... One mistake and we could be involved in a war."
A spokesman told reporters after the meeting there was consensus that there should be a proper debate on the issue to gain the people's understanding.
A Yomiuri Shimbun poll last month found that while 49 per cent of voters accept collective self-defence, only 27 per cent back the use of a constitutional reinterpretation to achieve it.
Both Emperor Akihito and Crown Prince Naruhito recently indicated at press conferences marking their birthdays that they were not in favour of revising the Constitution, which they saw as the basis for Japan's post-war peace and prosperity.
Mr Abe told Parliament last month that he planned to allow collective self-defence through a constitutional reinterpretation using a Cabinet decision. The LDP-led coalition controls both houses of Parliament.
Although Mr Abe gave no date for his proposed Cabinet decision, it was assumed he would try to do so after a government-appointed panel releases its report on the issue next month, and before the current session of Parliament ends on June 22.
Mr Abe is said to hope the policy shift will boost Japan's security ties with the United States and serve to check China's arms build-up and increasing maritime activities.
But the Mainichi Shimbun daily disagreed.
"We worry that without being able to remove international concerns over Japan's ties with China, South Korea and even the US, enabling collective self-defence for Japan only further raises tensions in the region," the daily said in an editorial last month.
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