Japan's nuclear agency wants to raise the severity level of a radioactive water leak at the Fukushima plant from one to three on an international scale, the BBC reports.
Highly radioactive water was found to be leaking from a storage tank into the ground at the plant on Monday.
It was first classified as a level one incident on the International Nuclear and Radiological Event Scale (Ines).
But the Nuclear Regulation Authority proposes elevating it to level three on the seven-point scale.
This week is the first time that Japan has declared an event on the Ines scale since the 2011 earthquake and tsunami.
The move was announced in a document on the agency's website and was subsequently approved at a weekly meeting of the regulatory body.
The March 2011 tsunami knocked out cooling systems to the reactors at the plant, three of which melted down.
Water is now being pumped in to cool the reactors but this means that a large amount of contaminated water has to be stored on site.
There have been leaks of water in the past but this one is being seen as the most serious to date, because of the volume - 300 tonnes of radioactive water, according to Tepco - and high levels of radioactivity in the water.