Image copyright Reuters Image caption The leak is thought to have occurred after a storage tank overflowed

Around 100 tonnes of highly radioactive water have leaked from a storage tank at Japan's Fukushima nuclear plant, operator Tokyo Electric (Tepco) says.

The toxic water may have overflowed after a valve was left open by mistake, Tepco said.

However the water was unlikely to have reached the ocean, the operator added.

The plant, which was damaged by an earthquake and tsunami in 2011, has faced multiple problems including leaks and power cuts since the disaster.

The latest leak is the most serious since August, when the plant leaked 300 tonnes of water, prompting Japan's nuclear agency to raise the incident's alert level.

'Contaminated earth'

The water from Wednesday's leak was radioactive, with a reading of 230 million becquerels per litre of radioactive isotopes, Tepco spokesman Masayuki Ono told reporters.

A becquerel is a unit used to measure radioactivity. WHO guidance advises against drinking water with radioactivity levels higher than 10 becquerels per litre.

Tepco says the radioactive water overflowed from a storage tank on Wednesday, but the leak was not discovered for several hours, the BBC's Rupert Wingfield-Hayes in Tokyo reports.

Previous Fukushima problems 9 Oct 2013: Six workers are accidentally doused in radioactive water

Six workers are accidentally doused in radioactive water 7 Oct 2013: A plant worker accidentally switches off power to pumps used for cooling damaged reactors

A plant worker accidentally switches off power to pumps used for cooling damaged reactors 3 Oct 2013: Tepco says there is a radioactive water leak after workers overfill a storage tank

Tepco says there is a radioactive water leak after workers overfill a storage tank 21 Aug 2013: Japan's nuclear agency upgrades Fukushima alert level

Japan's nuclear agency upgrades Fukushima alert level 20 Aug 2013: Tepco says 300 tonnes of radioactive water has leaked from a storage tank into the ground

Tepco says 300 tonnes of radioactive water has leaked from a storage tank into the ground July 2013: Tepco for the first time admits radioactive water is going into the sea

Tepco for the first time admits radioactive water is going into the sea June 2013: Tepco says radioactive water leaking from a storage tank to the ground

Tepco says radioactive water leaking from a storage tank to the ground April 2013: Tepco suspects a fresh radioactive water leak at Fukushima

Tepco suspects a fresh radioactive water leak at Fukushima March 2013: Tepco suspects a rodent may have been behind a power cut that shut down cooling systems

Tepco suspects a rodent may have been behind a power cut that shut down cooling systems Dec 2011 Contaminated water leaks from a treatment system, caused by a crack in the foundation

The operator says the leak occurred when contaminated water was accidentally pumped into a large storage tank that was already full, our correspondent adds.

"We apologise for worrying the public with such a leak," Mr Ono said. "Water is unlikely to have reached the ocean as there is no drainage in that tank area."

"We are now in the process of recovering the leaked water and the earth it has contaminated," he added.

On 11 March 2011, an earthquake and tsunami crippled the plant. Waves knocked out cooling systems for the reactors, leading to meltdowns at three of them.

Water is being pumped in to cool the reactors. However, this creates large amounts of contaminated water that must be stored securely.

The Fukushima nuclear power plant suffered a number of setbacks last year, including worker errors and a series of toxic water leaks that have lead to concerns contaminated water is mixing with groundwater that is flowing into the sea.