Japan's government is moving towards writing an emergency exception to its ban on nuclear weapons entering the country into its revised national security documents, a step that would mark the biggest shift in the country's nuclear policy in decades.

A senior government official said on 11 November that the administration of Prime Minister Sanae Takaichi intends to allow nuclear weapons into Japan in certain emergency situations, and to include that exception in three key security documents due for revision by the end of the year, according to reports of the remarks.

What the exception would allow

The exception could allow US nuclear-armed ships to make emergency port calls in Japan if the government determines that the country could not otherwise be adequately defended. It would apply to the third of Japan's Three Non-Nuclear Principles, which commit the country not to possess, not to produce and not to permit the introduction of nuclear weapons.

The current security documents state that Japan's policy of adhering to the principles "will remain unchanged". The proposal now under discussion would keep the principles themselves in place while spelling out, for the first time in an official document, the circumstances in which an exception could be made.

The idea draws on a position set out in parliament in March 2010 by Katsuya Okada, then foreign minister, who said that if Japan's safety could not be guaranteed without allowing a temporary port call by a US vessel carrying nuclear weapons, the government should stake its fate on the decision and explain it to the public. Successive governments have endorsed that view, but it has never been written into the security strategy.

Divisions inside the ruling camp

Takaichi, a security hawk, has called the principle of not allowing nuclear weapons into Japan "unrealistic", given Japan's reliance on US extended nuclear deterrence. She has also said the principles remain government policy, and has rejected nuclear sharing, under which US nuclear weapons would be stationed in Japan in peacetime and operated jointly in wartime, as "unacceptable".

The plan to spell out an exception formally has nevertheless met resistance inside her own Liberal Democratic Party, and the ruling parties only began working-level talks on revising the three documents on 10 November. The LDP's coalition partner, the Japan Innovation Party, is pressing for a review of the ban on bringing in nuclear weapons, while the LDP omitted the issue from its own recommendations in June.

No final wording has been agreed, and neither the definition of an emergency nor the procedure for approving a port call has been announced. The government aims to complete the revisions to the National Security Strategy, the National Defense Strategy and the Defense Buildup Program by the end of the year, leaving weeks of negotiation over language that will be watched closely in Washington, Beijing and Seoul, as well as in the Japanese port cities that host US forces.