The Nuclear Safety Commission (NSC) of Japan has said the crisis level at the Fukushima Daiichi plant has been raised to the highest level of seven.
The decision to raise the threat level was made after radiation of up to 630,000 terabequerels was estimated to have leaked from the plant.
NSC said the rating is a preliminary assessment and requires further evaluation by technical specialists.
Japanese Prime Minister Naoto Kan said radiation leaks at the plant were declining and the plant operator Tokyo Electric Power (Tepco) is expected to take up measures to bring the facility under control.
The amount of discharged radioactive materials is approximately 10% of the Chernobyl accident in 1986, which was also rated as level 7.