TOKYO — Japan has decided to raise its assessment of the accident at the crippled Fukushima Daiichi nuclear power plant to the worst rating on an international scale, putting the disaster on par with the 1986 Chernobyl explosion, the Japanese nuclear regulatory agency said on Tuesday.

The decision to raise the alert level to 7 from 5 on the scale amounts to an admission that the accident at the nuclear facility, brought on by the March 11 earthquake and tsunami, is likely to have substantial and long-lasting consequences for health and for the environment. Some in the nuclear industry have been saying for weeks that the accident released large amounts of radiation, but Japanese officials had played down this possibility.

The new estimates by Japanese authorities suggest that the total amount of radioactive materials released so far is equal to about 10 percent of that released in the Chernobyl accident, said Hidehiko Nishiyama, deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency.

Mr. Nishiyama stressed that unlike at Chernobyl, where the reactor itself exploded and fire fanned the release of radioactive material, the containments at the four troubled reactors at Fukushima remained intact over all.