The operator of the Fukushima Daiichi nuclear power plant ignored warnings that the complex was at risk of damage from a tsunami of the size that hit north-east Japan in March, and dismissed the need for better protection against seawater flooding, according to reports.

Tokyo Electric Power (Tepco) officials rejected "unrealistic" estimates made in a 2008 internal report that the plant could be threatened by a tsunami of up to 10.2 metres, Kyodo news agency said.

The tsunami that crippled backup power supplies at the plant on the afternoon of 11 March, leading to the meltdown of three reactors, was more than 14 metres high.

Evidence that the utility was unprepared for the tsunami, despite previous warnings, came as the firm announced that the manager of the Fukushima plant, Masao Yoshida, was being treated for an unspecified illness and would leave his post on Thursday.

The company refused to disclose the nature of Yoshida's illness, but said it was not related to his exposure to radiation during the nine months since the crisis began. "On doctors' advice, I have no choice but to be hospitalised for treatment," Yoshida, 56, reportedly said in a message to staff. "It breaks my heart to have to bid farewell in this way to all of the people with whom I have worked since the disaster."

Yoshida, who led the department overseeing the plant's management when the 2008 report was submitted, has been credited with preventing a more serious accident in March.

In the immediate aftermath of the tsunami, he approved the continued injection of seawater into one of the damaged reactors, despite being told to abandon the measure by Tepco officials. He was later reprimanded, but won praise from experts who said he had helped cool overheating fuel rods and prevent a worse disaster.

Yoshida had not spoken publicly about the accident until earlier this month, when he told reporters that the situation at Fukushima Daiichi had improved considerably and that the reactors would be brought to a safe state, known as cold shutdown, by the end of the year. But he added: "Several times during the first week of the crisis, I thought I was going to die."

The accident was triggered when seawater flooded power supply lines, disabling cooling systems and triggering a meltdown in three of Fukushima Daiichi's six reactors.

The 40-year-old plant was built on the assumption that the biggest tsunami that could be expected on the Fukushima coast would be 5.7 metres high. Even at that height, the 2008 report said, water levels onsite could exceed 15 metres.

Kyodo quoted Tepco sources as saying the plant might have been better prepared for the disaster had it taken the report seriously.

Greenpeace, meanwhile, called on Japan not to restart nuclear reactors taken offline for stress tests and maintenance checks until it improves its disaster-response plans. It said simulation maps of potential accidents being used to devise emergency response efforts did not take into account accidents of the severity of the Fukushima disaster.

Greenpeace said Japanese government officials had conceded that the Speedi simulations were inadequate, as they are confined to low-level releases of radiation over a six-mile radius. Contamination from the Fukushima accident has spread over a much wider area.

The emergency response effort was "slow, chaotic and insufficient, and it appears that the government has learned nothing from it", said Junichi Sato, executive director of Greenpeace Japan. "These maps show that there is a strong risk of reactor restarts being pushed through without a proper, science-based assessment on the real risks being conducted, and without proper precautions being taken to protect the communities around the plants."

More than 80% of Japan's nuclear reactors will lie idle once Kansai Electric Power suspends operation of a reactor for inspection at a plant in western Japan on Friday. The move will leave all but 10 of the country's 54 reactors out of service.

The danger contamination poses to food supplies was underlined this week when officials in Fukushima confirmed that 9kg of a batch of contaminated rice had been sold to consumers this month. The discovery came soon after they banned shipments of another batch of rice containing excessive levels of radioactive caesium.

The rice, grown at three farms in the town of Date, contained up to 1,050 becquerels of caesium per kg, compared with the government-set upper limit of 500 becquerels. In response, the government imposed a ban on Tuesday on rice shipments from the area, while local officials said they were trying to trace the consumers.