Water that the Japanese government is planning to release into the Pacific Ocean from the crippled Fukushima Dai-Ichi nuclear plant contains radioactive material well above legally permitted levels, according to the plant’s operator and documents seen by The Telegraph.

The government is running out of space to store contaminated water that has come into contact with fuel that escaped from three nuclear reactors after the plant was destroyed in the March 2011 earthquake and tsunami that struck north-east Japan.

Its plan to release the approximately 1.09 million tons of water currently stored in 900 tanks into the Pacific has triggered a fierce backlash from local residents and environmental organisations, as well as groups in South Korea and Taiwan fearful that radioactivity from the second-worst nuclear disaster in history might wash up on their shores.

Tokyo Electric Power Co., (Tepco) which runs the plant, has until recently claimed that the only significant contaminant in the water is safe levels of tritium, which can be found in small amounts in drinking water, but is dangerous in large amounts.

The government has promised that all other radioactive material is being reduced to “non-detect” levels by the sophisticated Advanced Liquid Processing System (ALPS) operated by the nuclear arm of Hitachi Ltd.

Documents provided to The Telegraph by a source in the Japanese government suggest, however, that the ALPS has consistently failed to eliminate a cocktail of other radioactive elements, including iodine, ruthenium, rhodium, antimony, tellurium, cobalt and strontium.

Hitachi declined to comment on the reports on the performance of its equipment. The Japanese government did not reply to multiple requests for comment.

A restricted document also passed to The Telegraph from the Japanese government arm responsible for responding to the Fukushima collapse indicates that the authorities were aware that the ALPS facility was not eliminating radionuclides to “non-detect” levels.

That adds to reports of a study by the regional Kahoko Shinpo newspaper which it said confirmed that levels of iodine 129 and ruthenium 106 exceeded acceptable levels in 45 samples out of 84 in 2017.

Iodine 129 has a half-life of 15.7 million years and can cause cancer of the thyroid; ruthenium 106 is produced by nuclear fission and high doses can be toxic and carcinogenic when ingested.

In late September, Tepco was forced to admit that around 80 per cent of the water stored at the Fukushima site still contains radioactive substances above legal levels after the Ministry of Economy, Trade and Industry held public hearings in Tokyo and Fukushima at which local residents and fishermen protested against the plans.

Tepco has now admitted that levels of strontium 90, for example, are more than 100 times above legally permitted levels in 65,000 tons of water that has been through the ALPS cleansing system and are 20,000 times above levels set by the government in several storage tanks at the site.

Dr Ken Buesseler, a marine chemistry scientist with the US Woods Hole Oceanographic Institution, said it was vital to confirm precisely what radionuclides are present in each of the tanks and their amounts.

“Until we know what is in each tank for the different radionuclides, it is hard to evaluate any plan for the release of the water and expected impacts on the ocean”, he told The Telegraph.

Experts agree that the danger posed by any release depends on the concentrations of radionuclides and the subsequent contamination of fisheries products.

The presence of strontium in the bones of small fish that might be consumed by humans could be a cause of major concern. If ingested by humans, strontium 90 builds up in teeth and bones and can cause bone cancer or leukaemia.

Shaun Burnie, a nuclear specialist with Greenpeace, also disputes Tepco’s claims that tritium is effectively harmless.

“Its beta particles inside the human body are more harmful than most X-rays and gamma rays”, he said, adding that there “are major uncertainties over the long-term effects posed by radioactive tritium that is absorbed by marine life and, through the food chain, humans.

“The planned release of billions of becquerels by Tepco cannot be considered an action without risk to the marine environment and human health”.

The Japanese authorities have been caught off-guard by the strength of local opposition to the plan to release the contaminated water into the ocean, but have so far not dropped their plans.