Japan's atomic regulator Wednesday ordered a halt to plans to restart an experimental reactor because of safety concerns, as a strong anti-nuclear mood grips the nation.

Local media said the move by the Nuclear Regulation Authority (NRA) would make it impossible for the Monju Prototype Fast Breeder Reactor to begin working again this fiscal year.

The NRA has been examining Monju since it was revealed last year that appropriate safety checks had been skipped on nearly 10,000 pieces of equipment at the plant, on the coast 350 kilometres (220 miles) west of Tokyo.

At a meeting Wednesday the watchdog approved a report that said the plant's operator was sloppy and had knowingly carried out safety checks inappropriately.

The report noted there had been "a degradation of the safety culture" at the plant and NRA chairman Shinichi Tanaka told the meeting: "The situation is quite serious".

Commissioner Kunihiko Shimazaki denounced operator Japan Atomic Energy Agency for ad-hoc responses every time the authorities pointed out faulty safety measures.

"The problem is that this kind of organisation exists and that we allow it to exist," Shimazaki said.

Japan Atomic Energy Agency will be given a chance to contest the decision before a formal order is issued. No date has yet been set.

Unlike regular light-water reactors that run on uranium, fast-breeders use a mix of plutonium and uranium, including waste from conventional reactors, and generate or "breed" more plutonium than they consume.

Major industrialised nations initially rushed to develop the "dream reactors", but technical problems and fears over the proliferation of weapons-grade plutonium have led many to withdraw from the projects.

Most of Japan's atomic reactors are offline amid strong anti-nuclear sentiment in the aftermath of the 2011 emergency at Fukushima, caused by an earthquake and tsunami that swamped the plant.

Monju first reached criticality in 1994 but was shut down the following year amid public anger over the cover-up of a massive sodium leak and a fire.

The controversial reactor was restarted in May 2010 but was shut down again in August of that year following another accident. It has been offline since.