'You would walk through the factory and it was always covered in this thick layer of dust,' recalls John Grieves, a 65-year-old machine engineer, of his workplace from 1964 and 1969. 'As the sun shone through the windows you could seethe dust hanging in the air.'

What John then considered harmless dust, he knows now to have been killer asbestos. The factory made sheets, gutters and pipes out of a cement and asbestos mix. One of John's jobs was to maintain the hammers that used to pulverise the raw solid asbestos as it arrived in the factory in hessian sacks. 'We would often stand there knee-deep in asbestos dust while we changed the hammers in this shaft,' he says.

Two years ago a routine x-ray revealed that John had pleural plaques. These are internal scars on the lining of the lungs which can indicate asbestos exposure. They have no clinical symptoms but patients are often devastated, because of the risk of asbestos-related cancer.

'It was something I had expected for a long, long time and all of a sudden there it was,' says John, a father of three. 'It's all right for doctors to tell you there's nothing wrong but I've known a lot of people who have had has this and then died. It hit me pretty badly.'

John has had more bad news this month. His compensation claim, along with those of nine other pleural plaques sufferers, is being challenged by their employers' insurers at the High Court in Manchester. If the insurers are successful, there will be no compensation for pleural plaques in British courts.

The condition might indicate that an individual has been exposed to asbestos, argues Dominic Clayden, director of technical claims at Norwich Union, but also reveal exposure to sand quartz minerals or even talc. 'Plaques aren't a disease and they do not cause symptoms,' he argues. 'Furthermore, they do not develop into any other condition such as lung cancer or mesothelioma [the invariably terminal cancer of the lining of the lungs caused by asbestos]. The important distinction to make is that it is the exposure to asbestos that may lead to another condition, not the plaques themselves.'

So why should plaque sufferers receive compensation? David Mears, a 61-year-old joiner who worked with asbestos on a building site in Hull in 1976, found he had pleural plaques in March 2002. 'It is like being told you have cancer,' recalls David, who is married with three children. 'It was devastating.' His claim too is being challenged by his insurers.

It is not just people working on factory floors and building sites who are at risk. The Court of Appeal will consider another case this month on the cut-off time by which bosses should have done something to protect workers' families from such risks. Increasingly, the wives of former asbestos workers have developed mesothelioma as a result of washing work clothes. The appeal judges will hear the case of Teresa Maguire, who died in May after contracting the disease as a result of washing the overalls of her husband, Jimmy, a boilermaker in the Liverpool docks.

John Grieves is concerned about the health of his wife - who washed his asbestos-covered clothes for years - and of his eldest daughter, who was a baby at the time.

There are other reasons why pleural plaques sufferers will be hoping that the judge kicks out the insurers' challenge this month. 'The big issue here is that if someone is diagnosed with plaques, time starts running against them for limitation purposes,' says Anthony Coombs, a partner at Man chester law firm John Pickering and Partners. 'You then have three years to bring an action for any type of asbestos-related disease. The clock starts ticking.'

The insurers are quite open about their concerns at footing the bill for the asbestos crisis. Insurance premiums are based on current and future claims, explains Norwich Union's Dominic Clayden. 'Of course, if the present claims, trends and patterns for this condition are unchecked, there may be more and more claims for asymptomatic pleural plaques and premiums may have to rise to reflect that,' he says. In other words, it isn't just the insur ance industry that will be paying the price for these claims - we all will.

The actuarial profession reckons that the future UK cost of asbestos-related diseases will be £8 billion to £20bn, which represents 80,000-200,000 new claims over the next 30 years. It claims that half that cost will be met by the insurance industry. It is further ammunition in the propaganda war between lawyers, their clients and the insurers that is being waged around this month's test case.

The insurance industry and the actuarial profession blame lawyers who, they say, have been diverted from chasing ambulances to driving 'scan vans' in search of a new and lucrative form of compensation claims. On the other side of the Atlantic, lawyers fund mobile x-ray clinics that scour the country to screen workers and drum up asbestos-related lawsuits. It is estimated that these 'worried-well' claimants account for three-quarters of all asbestos compensation claims in the US.

The compensation for plaques is a relative pittance: a typical award is £5,000, with the right to return to court if a serious illness develops, or £10,000 as a 'full and final' settlement.

Anthony Coombs's firm specialises in asbestos claims and he has 200 pleural plaque clients. So does he have a scan van? 'No, I drive a Toyota,' he says. But the solicitor is aware of non-lawyer claims companies that have deployed mobile units to find new clients. They then try to sell the cases on to solicitors.'We always send them away,' he says.

The lawyers argue that the test case is a cynical attempt by the insurance industry to dodge its responsibility to injured people. A couple of years ago the law lords rejected another technical challenge by insurers, who then argued that mesothelioma was not due to any single asbestos exposure if the victim had more than one source of exposure.

'They think they have justice on their side this time because people aren't dying of pleural plaques but what is never mentioned is that none of this needed to have happened if the employer had taken basic safety precautions to prevent people being exposed,' says Colin Ettinger, president of the Association of Personal Injury Lawyers. 'I have no sympathy for the insurance industry - they have known that asbestos was dangerous for years.'

So what would David Mears say to the insurers who would assure him that his condition is harmless? 'Would they like to swap places with me?' he replies. 'What disturbs me is that the employers can admit liability and acknowledge that what they have done was wrong. Yet they try to walk away from it. Whereas I'm the innocent party and I'm left with this condition.'