Japanese authorities say treatment plants are fast running out of room to store sewage contaminated by radiation from the Fukushima nuclear plant.

Authorities believe radioactive material from the stricken plant seeped into sewage pipes, ending up at treatment facilities.

Toxic radioactive caesium has been found in sludge at a treatment plant in Fukushima, while radioactivity has also been found in sewage in Tokyo.

Normally, most of the sludge is recycled into cement after being incinerated into ash. But cement processors are refusing to accept the sludge from the stricken plant.

Sewage plants now warn they will run out of storage space in a few days.

Earlier, the Japanese government admitted it was unprepared for the scale of the Fukushima disaster which struck after a devastating earthquake and tsunami in March.

In an official report to be submitted to the UN's nuclear watchdog, Japan acknowledges there was insufficient communication between the government and the plant's operator.

Japan is now promising to separate its nuclear regulator from the ministry which promotes nuclear energy.

The disaster is rated a maximum seven on the international nuclear accident scale, the same level as the Chernobyl meltdown 25 years ago.