Tens of thousands of Russians living near the facility have been treated for the effects of radiation exposure for years.

Yuri Zolotov, deputy prosecutor general in the Urals region, told NTV television that an investigation showed that liquid radioactive waste had continuously been dumped from Mayak into the Techa river, which eventually flows into Siberia's major Ob river and on to the Arctic Ocean.

Vremya Novostei daily newspaper quoted Zolotov as saying radiation in the area exceeded safe levels by more than 200 percent. A formal criminal investigation was launched on Monday.

A similar investigation in 2003 led to Mayak's shut-down, but the plant was later reopened.

Ecology groups have long urged the government to shut the plant and welcomed the latest criminal investigation.

"But the main question now is whether this case would be seen through to a conclusion, whether the guilty would be punished and the plant's licence withdrawn," Vladimir Slivyak of EcoDefence ecology group said in a statement.

"Otherwise it would be a waste of time."

Mayak is one of Russia's biggest plants where nuclear waste generated by atomic power plants is processed to extract plutonium and prepare it for storage.