Residents who lived close to the damaged Fukushima nuclear plant are to be told their homes may be uninhabitable for decades, according to Japanese media reports. The Japanese prime minister, Naoto Kan, is expected to visit the area at the weekend to tell evacuees they will not be able to return to their homes, even if the operation to stabilise the plant's stricken reactors by January is successful.

Kan's announcement will be the first time officials have publicly recognised that radiation damage to areas near the plant could make them too dangerous to live in for at least a generation, effectively meaning that some residents will never return to them.

A Japanese government source is quoted in local media as saying the area could be off-limits for "several decades". New data has revealed unsafe levels of radiation outside the 12-mile exclusion zone, increasing the likeliness that entire towns will remain unfit for habitation.

The exclusion zone was imposed after a series of hydrogen explosions at the plant following the earthquake and tsunami in March.

The government had planned to lift the evacuation order and allow 80,000 people back into their homes inside the exclusion zone once the reactors had been brought under control. Several thousand others living in random hotspots outside the zone have also had to relocate.

However, in a report issued over the weekend the science ministry projected that radiation accumulated over one year at 22 of 50 tested sites inside the exclusion zone would easily exceed 100 millisieverts, five times higher than the safe level advised by the International Commission on Radiological Protection. "We can't rule out the possibility that there will be some areas where it will be hard for residents to return to their homes for a long time," said Yukio Edano, chief cabinet secretaryand face of the government during the disaster. "We are very sorry."

Edano refused to say which areas were on the no-go list or how long they would remain uninhabitable, adding that a decision would be made after more radiation tests have been conducted.

The government has yet to decide how to compensate the tens of thousands of residents and business owners who will be forced to start new lives elsewhere. The state has hinted that it may buy or rent land from residents in unsafe areas, although it has not ruled out trying to decontaminate them.

Futaba and Okuma, towns less than two miles from the Fukushima plant, are expected to be among those on the blacklist. The annual cumulative radiation dose in one district of Okuma was estimated at 508 millisieverts, which experts believe is high enough to increase the risk of cancer. More than 300 households from the two towns will be allowed to return briefly to their homes next week to collect belongings. It will be the first time residents have visited their homes since the meltdown.

The plant's operator, Tokyo Electric Power, is working to bring the three crippled reactors and four overheating spent fuel pools to a safe state known as "cold shutdown" by mid-January.

Last week the company estimated that leaks from all three reactors had dropped significantly over the past month. But signs of progress at the plant have been tempered by widespread contamination of soil, trees, roads and farmland.

Experts say that while health risks can be lowered by measures including the removal of layers of topsoil, vulnerable groups such as pregnant women and children should avoid even minimal exposure.

"Any exposure would pose a health risk, no matter how small," Hiroaki Koide, a radiation specialist at Kyoto University, told Associated Press. "There is no dose that we should call safe."

Any government admission that residents will not be able to return to their homes will be closely monitored in Japan.

Suspicions persist that the authorities privately acknowledged this situation several months ago. In April, Kenichi Matsumoto, a senior adviser to the cabinet, quoted Kan as saying that people would not be able to live near the plant for "10 to 20 years". Matsumoto later claimed to have made the remark himself.

(By Justin McCurry, Guardian.co.uk, 22/08/2011)