Japan is to issue its gravest warning about the state of the wrecked Fukushima Daiichi nuclear power plant since the facility suffered a triple meltdown almost two and a half years ago.

The new warning, expected on Wednesday, comes only a day after the nuclear watchdog assigned a much lower ranking when the plant's operator, Tepco, admitted about 300 tonnes of highly toxic water had leaked from a storage tank at the site.

The Nuclear Regulation Authority has now said it will dramatically raise the incident's severity level from one to three on the eight-point scale used by the International Atomic Energy Agency (IAEA) for radiation releases. Each single-digit increase in the scale actually represents a tenfold increase in the severity of a radiological release, according to the IAEA.

The NRA on Tuesday classified the leakage only as an "anomaly" on the IAEA scale but now considers it a "serious incident".

The leak is the single most dangerous failure at the plant since the 2011 meltdown, which warranted the maximum level of seven on the severity scale, putting it on a par with the Chernobyl disaster 25 years earlier.

"Judging from the amount and the density of the radiation in the contaminated water that leaked ... a level three assessment is appropriate", the NRA said in a document posted on its website on Wednesday.

Tepco has admitted it has yet to identify the cause of the leak, in which highly radioactive water appears to have breached a steel storage tank and seeped into the ground. The leak from the tank, which can hold up to 1,000 tonnes of water, has yet to be stemmed, according to Japanese media reports.

The incident is separate from additional water leaks of up to 300 tonnes a day recently reported by Tepco.

Those spillages involve groundwater that is leaking into the nearby Pacific ocean. Tepco said there was no evidence that water from the storage tank had found its way into the sea. The utility said radioactive matter, including caesium and strontium, had seeped into soil, which may have to be dug up and removed.

Puddles near the faulty tank are so contaminated that a person standing 50 centimetres away would in one hour receive five times the annual dose limit for Japanese nuclear workers. Initial readings showed that radiation levels in one puddle were 100 millisieverts an hour.

The leak underlined the risks being taken by workers at the site; after 10 hours anyone in proximity to the contaminated water would develop radiation sickness, with symptoms including nausea and a drop in white blood cells.

The new warning is the most serious since the plant was struck by a powerful tsunami on 11 March 2011. The wave, triggered by a magnitude 9.0 earthquake off Japan's north-east coast, wrecked the plant's back-up power supply and sent three of its six reactors into meltdown.

Tepco is using huge quantities of water to cool melted fuel rods lying deep inside the wrecked reactors, but has failed to prevent contaminated water leaking into the ground and the sea. Contaminated water is stored in about 1,000 tanks that have been built about 500 yards from the shoreline.

The prime minister, Shinzo Abe, recently indicated he had lost confidence in Tepco's ability to deal with the water crisis.

Abe, a supporter of nuclear energy, said the government would intervene to help the utility stem the leaks and begin the long and costly process of decommissioning the plant.

The new leaks have fuelled public anger over Tepco's handling of contaminated water and caused concern overseas.

Asiana Airlines, South Korea's second-biggest carrier, said it would cancel four chartered flights between Seoul and Fukushima in October out of concern over radiation leaks.