TOKYO – Criticism of the Japanese government’s handling of the crisis at a radiation-spewing nuclear power plant increased Saturday, with a new poll indicating three-quarters of people disapprove, and a key adviser quitting in protest.

A Kyodo News service poll released Saturday showed that Prime Minister Naoto Kan’s support ratings were plunging.

The poll reported that 76 percent of the respondents think Kan is not exercising sufficient leadership in handling the quake, tsunami and nuclear triple crisis, up from 63.7 percent in the previous survey in late March.

It also showed 23.6 percent of respondents think Kan should resign immediately, up from 13.8 percent in the previous survey.

The telephone survey, conducted by Kyodo News on Friday and Saturday, did not give a margin of error.

Toshiso Kosako, a professor at the University of Tokyo’s graduate school and an expert on radiation exposure, announced late Friday that he was stepping down as a government adviser over what he lambasted as unsafe, slipshod measures.

Prime Minister Naoto Kan appointed Kosako after the magnitude-9.0 earthquake and tsunami that struck northeastern Japan on March 11. The disaster left 26,000 people dead or missing and damaged several reactors at the Fukushima Dai-ichi nuclear power plant — setting off the world’s worst nuclear crisis since Chernobyl in 1986.

In a tearful news conference, Kosako said he could not stay and allow the government to set what he called improper radiation limits of 20 millisieverts an hour for elementary schools in areas near the plant.

“I cannot allow this as a scholar,” he said. “I feel the government response has been merely to bide time.”

Kosako also criticized the government as lacking in transparency in disclosing monitoring of radiation levels around the plant, and as improperly raising the limit of radiation exposure levels for workers at Fukushima Dai-ichi, according to Kyodo News agency.

The prime minister defended the government’s response as proper.

“We welcome different views among our advisers,” Kan told parliament Saturday in response to an opposition legislator’s questions.

A government advisory position is highly respected in Japan, and it is extremely rare for an academic to resign in protest of a government position.

The science and education ministry has repeatedly defended the 20-millisievert safety limit for radiation exposure, saying that efforts are under way to bring the limit down to 1 millisievert. Some people have expressed concerns, noting that children are more vulnerable to radiation than adults.

Workers in the U.S. nuclear industry are allowed an upper limit of 50 millisieverts per year. A typical individual might absorb 6 millisieverts a year from natural and man-made sources such as X-rays.

Radiation specialists say cumulative doses of 500 millisieverts raise cancer risks. Evidence is less clear on smaller amounts, but in theory, any increased radiation exposure raises the risk of cancer.