The governor of Japan's southern prefecture of Okinawa has maintained his opposition to approving ground improvement work as part of a project to relocate a US air base within the prefecture.

Tamaki Denny on Monday decided to defy a high court ruling that orders the prefecture to give its go-ahead for the work at an offshore site in Henoko, Nago City.

Monday was the deadline for Tamaki to decide whether or not to approve the work.

The central government plans to relocate the US Marine Corps Futenma Air Station to the offshore site from a densely populated area in Ginowan City.

The government now plans to take the unprecedented step of overriding a decision made by local authorities.

Tamaki is considering contesting the high court's ruling and filing an appeal at the Supreme Court.