By Amitabha Bhattasali
BBC News, Calcutta
The NGO study was conducted throughout West Bengal.
Alarming levels of toxic mercury have been found in fish in the Indian state of West Bengal, experts say.
They say that popular varieties of fish used for human consumption are contaminated both in the water and when on sale in the markets.
The findings were made by two Indian non-governmental organisations who carried out research across the state.
The state government has yet to respond to the findings, but it has blamed heavy industry for the contamination.
The study was undertaken by two Indian NGOs - Toxics Link and Disha - throughout West Bengal at water sources and in markets.
"The study shows that popular varieties of fish contain mercury concentrates in excess of the Prevention of Food Adulteration (PFA) Act standards of 0.5 parts per million," the report says.
Out of the 264 samples tested at an accredited laboratory, 129 of the fish showed methyl mercury levels (a more poisonous form of mercury) exceeding the PFA stipulations.
"The mercury contamination is highest at Jharkhali in the Sunderbans (a mangrove swamp in the south-east of West Bengal) but high levels of mercury were also found in other places of the coastal belt, including the popular sea resort of Digha," said Santanau Chacraverti, one of the lead researchers of the study.
"Places near (state capital) Calcutta also show the presence of a high level of mercury. However, samples collected from north Bengal - at the foothills of the Himalayas - hardly contained the metal at all."
His team also examined fish consumption patterns to assess risk. It was found that the protein-rich fish were more likely to affect pregnant mothers, foetuses and young children.
"High levels of methyl mercury lead to neurotoxicity - it affects the brain's development, stunts psychological development and can cause serious mental disorders over a gradual period of time," said Dr Sisir Das, a neurologist based in Calcutta.
"IQ levels could drop and an individual's mental stability is jeopardised. It can also cause swelling of the lungs and ultimately lead to death."
West Bengal's pollution control board said that it was aware that thermal power stations and other industries that burn coal cause mercury contamination.
However a spokesman declined to comment on the recent study.
Evidence of the mercury contamination was one of the main issues discussed in a recently concluded UN Environment Programme (UNEP) meeting in Nairobi, Kenya.