Japan's nuclear safety agency has raised the severity rating of the crisis at its nuclear plant to the highest level, on a par with the 1986 Chernobyl disaster.
An official with the Nuclear Safety Commission of Japan, speaking on national television, said the rating was raised from five to seven.
The official said the amount of radiation leaking from the Fukushima Dai-ichi nuclear plant was around 10pc of that in the Chernobyl accident in the former Soviet Union.
Level 7 signifies a "major accident" with "wider consequences" than the previous level, according to the standards scale.
"We have upgraded the severity level to seven as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean," said Minoru Oogoda of Japan's Nuclear and Industrial Safety Agency (Nisa).
Nisa officials said one of the factors behind the decision was that the total amount of radioactive particles released into the atmosphere since the incident had reached levels that applied to a level seven incident.
The action lifts the rating to the highest on an international scale designed by a group of experts in 1989 and is overseen by the International Atomic Energy Agency.
In Chernobyl, in the Ukraine, a reactor exploded on April 26 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 30km around the plant was declared uninhabitable, although some plant workers still live there for short periods and a few hundred other people have returned despite government encouragement to stay away.
Meanwhile, setbacks continued at Japan's tsunami-stricken nuclear power complex, with workers discovering a small fire near a reactor building at 8.38pm BST on Monday. The fire was extinguished quickly, the plant's operator said.
Tokyo Electric Power Co (Tepco) which operates the disabled Fukushima Dai-ichi nuclear power plant, said the fire at a box that contained batteries in a building near the No 4 reactor was discovered at about 6.38am on Tuesday, local time, and was put out seven minutes later.