Workers fighting a legal battle against Rio Tinto feel a breakthrough is near, but some may not live to see their case resolved

It is a matter of some embarrassment to Arthur Gray that he recently fell over in the company of the Deputy Prime Minister.

It is a matter of some embarrassment to Arthur Gray that he recently fell over in the company of the Deputy Prime Minister.

The 56-year-old was at one of John Prescott's constituency surgeries two weeks ago, explaining to his MP why he believes the tin smelting plant on the north bank of the river Humber - where he worked for 21 years - had nearly killed him, when he seemed to stumble over a chair leg.

"Watch your step," Mr Prescott told him, and at the time Mr Gray just put it down to swelling in his legs, a consequence of the chemotherapy he had undergone for throat cancer. With hindsight, the fall may have indicated something more serious - a brain tumour diagnosed by Mr Gray's doctors only last Tuesday.

The timing of Mr Gray's setback, seven years after he recovered from cancer of the right lung, was wretched because it coincided with an unanticipated breakthrough in a three-year campaign to win compensation from Rio Tinto plc, the multinational mining company which operated the smelting works west of Hull - the world's largest before it was decommissioned in 1991.

The company agrees that if lawyers representing the families of 200 former workers - some of whom are now dead - can prove that exposure to chemicals such as arsenic, lead, cadmium and radioactive polonium-210 has been the cause of illness, they will admit negligence, pay compensation and workers' legal costs, and waive the normal three-year limit on bringing such cases to the High Court.

Industry analysts are surprised by the concession. It is an unprecedented move for a company that is not in the habit of conceding negligence. Its timing suggests they have something to fear, according to the FT's Global Water Report, which has monitored workers' pursuit of compensation.

It may be significant that Rio Tinto agreed to the compensation procedure the day before the senior regional solicitor to the Transport and General Workers' Union (TGWU), David Russell, was due to publish a 500-page dossier based on his investigations into the plant, which followed the 1997 death from lung cancer of a 67-year-old union member who believed he had been poisoned by carcinogens from the plant.

For years, locals have suspected they have paid a heavy price for the plant, which at its peak employed 1,000 people and produced 15 per cent of the western world's tin as well as copper, lead and precious metals. It has never been explained why a cluster of 12 children in three outlying villages suffered cancers and why seven of them died in the mid-1980s.

It was not until last November that Mr Russell appeared to win a breakthrough when the company - which had told MPs that it operated "at all times under the normal regulations then in force" - first admitted to Kevin McNamara, MP for Kingston upon Hull North, that "contrary to previous understanding, there were regulatory infringements on site" relating to emissions. This, the company conceded, had emerged as investigations were carried out to "provide answers to Mr Russell".

A particularly significant piece of Mr Russell's evidence is a 1990 Health and Safety Executive (HSE) report, released last year, which concluded that the smelter was being operated in breach of environmental regulations and posed a potentially major health hazard to employees. The provision of radiation protection was "ill considered" and "adequate precautions" were "not being taken to ensure employees ... are not exposed to unacceptable levels of arsenic dust", says the report, which has been seen by The Independent. The plant closed a year after publication of the report.

The HSE investigation followed an improvement notice that was served on the plant in 1988. It told the company "to ensure employees and possibly others are not exposed to unacceptable levels of arsenic dust and fumes".

Mr Russell also has a 1987 Ministry of Agriculture report, which shows that levels of arsenic in fish in the Humber were twice as high as anywhere else in the UK. His work has also been the spur for pressure on the company from Westminster: three parliamentary motions highlighting his research have been tabled in the past 12 months.

Rio Tinto insists it has nothing to fear from compensation tribunals. "We've never seen subject matter that proves Russell's medical claims. We simply want an outcome. No socially responsible corporation wants accusations to be levelled on a long-term basis," said a spokeswoman.

Establishing a medical link between emissions and deaths is not a foregone conclusion.

Mr Gray smoked heavily for 40 years and it seems he will only have a case if his medical records - now sought from Rio Tinto by Mr Russell - bear out his claim that doctors treating his lung cancer told him they were "looking to the chest of a 70-year-old" and that his tumour was "unusual". He worked from 1970 to 1991 in the panning sheds and furnace rooms of the plant from which, he claims, men ran out when the chemical processes used to tap metals from ore at the plant gave off fumes. Face masks were not compulsory in the factory sheds, he claims.

Alan Nicholls has also contributed evidence to the investigation. He farms land which overlooked the plant's stack, five miles away, and took his sheep to veterinary laboratories in Leeds, Thirsk, Manchester and Lincoln to find an explanation of why, throughout the 1970s and 1980s, abattoirs were rejecting the livers of 250 out of his yearly herd of 7,000 sheep because they were damaged.

"It happened from November to February when the weather was more atmospheric and the stack plumes drifted over," he said. Independent scientific tests have shown his land to contain seven times the normal level of copper, and the copper and lead traces in his sheep's milk to be 10 times higher than normal.

Rio Tinto points to three independent investigations that failed to establish a link between the plant and cancers. But the investigations are not exactly conclusive. One, carried out by Leeds University, identified "unexplained and unexpected numbers of tumours" and called for further investigation.

Another, by the Scottish University Research and Reactor Centre in 1990, did not rule out a link and reported that workers were being exposed to unsafe levels of radioactivity. It provoked an angry exchange with the National Radiological Protection Board, which was responsible for monitoring the plant and said the report was "inadequately substantiated".

Mr Gray is due to undergo further medical tests on Tuesday and continues to hope that he will live to see the compensation he believes he is owed.