High levels of a toxic radioactive isotope have been found in groundwater at Japan's Fukushima nuclear plant, its operator says, according to BBC News.

Tokyo Electric Power Company (Tepco) said tests showed strontium-90 was present at 30 times the legal rate. The radioactive isotope tritium has also been detected at elevated levels.

The plant, crippled by the 2011 earthquake and tsunami, has recently seen a series of water leaks and power failures. The tsunami knocked out cooling systems to the reactors, which melted down.

Water is now being pumped in to the reactors to cool them but this has left Tepco with the problem of how to safely store the contaminated water.

There have been several reports of leaks from storage tanks or pipes.

Strontium-90 is formed as a by-product of nuclear fission. Tests showed that levels of strontium in groundwater at the Fukushima plant had increased 100-fold since the end of last year, Toshihiko Fukuda, a Tepco official, told media.

Tritium, commonly used in glow-in-the-dark watches, was found at eight times the allowable level.

A Tepco official said that samples from the sea showed no rise in either substance and the company believed the groundwater was being contained by concrete foundations.

But the discovery is another set-back for Tepco's plan to pump groundwater from the plant into the sea, BBC News reported.