When is an aircraft carrier not an aircraft carrier?

It looks like an aircraft carrier, operates like an aircraft carrier and now Japan is ready to admit to what its suspicious neighbours have suspected all along — that its largest warship is in fact an aircraft carrier.

The government of Shinzo Abe is investigating ways to upgrade the 248m JS Izumo, designated a “helicopter carrying destroyer”, so that it can also launch fixed-wing fighter jets.

The change, which is intended to counter the growing power of the Chinese navy, will add to anxieties about an arms race in the waters of East Asia.

The Izumo is as large as Japan’s wartime carriers, and bigger than the decommissioned British carrier HMS Illustrious. Yet despite its wide expanse of unbroken deck, Japan has always insisted that it is no more than a helicopter platform, which does not violate the terms of its “pacifist” constitution.

Aircraft carriers are banned under the present interpretation of the constitution because they provide the ability to launch attacks on other nations. When the Izumo was commissioned in 2015, Japan emphasised its role in peacekeeping and humanitarian missions.

According to Japanese media, the government will look for ways of designating it a “mother ship”, rather than an “attack aircraft carrier”, sidestepping the constitutional difficulty.