Officials of the Tokyo Electric Power Company (TEPCO) said on Friday they detected 400,000 becquerels per liter of beta ray-emitting radioactive substances - including strontium - at the site on Thursday, a level 6,500 times higher than readings taken on Wednesday, NHK World reported.

The storage tank leaked over 300 tons of contaminated water in August, some of which is believed to have found its way into the sea through a ditch.

The well in question is about 10 meters from the tank and was dug to gauge leakage.

TEPCO said the findings show that radioactive substances like strontium have reached the groundwater. High levels of tritium, which transfers much easier in water than strontium, had already been detected.

Officials at TEPCO said they will remove any contaminated soil around the storage tank in an effort to monitor radioactivity levels of the water around the well.

The news comes after it has been reported a powerful typhoon which swept through Japan led to highly radioactive water near the crippled nuclear power plant being released into a nearby drainage ditch, increasing the risk of it flowing into the sea.

On Wednesday TEPCO said it had detected high levels of radiation in a ditch leading to the Pacific Ocean, and that it suspected heavy rains had lifted contaminated soil.

BA/BA