A leak of highly radioactive water at the crippled Fukushima nuclear power plant will not affect Tokyo's bid to host the 2020 Olympics, Tokyo Governor Naoki Inose said Friday.

About 300 300,000 litres of contaminated water was found to have leaked from one of the tanks at the wrecked Fukushima Dai-ichi plant on Monday.

The leak is the fifth — and worst — since last year, but Inose said it is not a threat to Tokyo's Olympic plans.

"Regarding food and water in Tokyo, there is absolute safety and the data is available," Inose said. "As far as hosting the Games, the situation in Fukushima will not affect Tokyo."

Tokyo, Istanbul and Madrid are bidding to host the 2020 Olympics.

The IOC will select the host city by secret ballot on Sept. 7 in Buenos Aires, Argentina.

Inose said data shows that radiation levels in Tokyo are the same as London, Paris and New York.

Fukushima is 250 kilometres north of Tokyo.

The plant suffered multiple meltdowns following a massive earthquake and tsunami in March 2011 — a Level 7 "major accident" and the worst since Chornobyl in 1986.

Japan is pledging tighter nuclear safety.

Most of Japan's nuclear plants remain closed after they were shut down for safety checks after the Fukushima disaster.