Image caption The plant was badly damaged by the earthquake and tsunami of March 2011

Plans to decontaminate six towns and villages close to Japan's Fukushima nuclear plant have to be delayed by up to three more years, officials say.

The clean-up of the exclusion zone around the crippled plant was initially due to be completed by next March.

More than 90,000 people remain unable to return home.

Fukushima has been hit by a series of toxic water leaks in recent months. The latest contamination was reported on Sunday after unexpectedly heavy rain.

Water with high levels of the toxic isotope Strontium-90 overflowed containment barriers around water tanks, operator Tepco said.

The tanks are being used to store contaminated cooling water from reactors damaged by the earthquake and tsunami of March 2011.

Cooling systems for reactors were knocked out, causing meltdowns at three of them.

Previous Fukushima problems 21 Oct: Radioactive water overflows a containment barrier after heavy rain

Radioactive water overflows a containment barrier after heavy rain 7 Oct 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 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 Japan's nuclear agency upgrades Fukushima alert level

Japan's nuclear agency upgrades Fukushima alert level 20 Aug 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 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 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 Tepco suspects a fresh radioactive water leak at Fukushima

Tepco suspects a fresh radioactive water leak at Fukushima March 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

On Monday Japan's environment ministry acknowledged the decontamination of towns around the plant is proving much more complicated than originally thought.

Tens of thousands of workers are engaged in the massive clean-up effort, removing millions of tonnes of topsoil and vegetation. But in the most highly contaminated areas work is yet to begin.

The government's latest prediction is that residents will be able to return home by 2017.

But the BBC's Rupert Wingfield-Hayes in Tokyo says many have already decided they will never go back.

Storms forecast

Ministry officials cited several reasons for the delay, including a lack of space for the waste from the decontamination work.

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

Some of the water has leaked from the tanks, pipes and damaged structures, leading to concerns that contaminated water is mixing with groundwater flowing into the sea.

On Sunday, the rainwater run-off - which becomes contaminated when it hits polluted surfaces areas - overflowed concrete barriers surrounding a group of tanks, reports citing the Tokyo Electric Power Company (Tepco) say.

Tepco says that in one area, readings of Strontium-90 were at 70 times the legal limit for safe disposal.

"Our pumps could not keep up with the rainwater. As a result, it flowed over some containment areas," Tepco spokesman, Yoshikazu Nagai, told Reuters news agency.

The company was quoted by Kyodo news agency as saying that some of the toxic water had seeped into the ground, but that it was unlikely to have flowed into the sea.

Strontium-90 is a radioactive by-product of the fission of uranium and plutonium inside a nuclear reactor. It is easily absorbed by the human body and is thought to be a cause of bone cancer.

More rains are expected in Japan, with meteorologists forecasting a typhoon to make landfall this week.