Defence ministers are wondering whether to stir the ashes of British military history to find out if service personnel used as human guinea pigs in chemical warfare experiments are now suffering from disease as a result.

More than 20,000 were put through test programmes in the laboratories of Porton Down, Wiltshire between 1916 and the present day. Indeed, the defence establishment conducted the world's longest programme of chemical warfare experiments on humans.

Poisons, including nerve gas and mustard gas, chemical weapons and protective drugs were tested on humans in myriad, often horrific, experiments. A full study would have to establish whether each type of chemical caused later ill-health.

But Porton's legacy is about more than a scientific inquiry. This issue is also about people's confidence in the ability of the Ministry of Defence (MoD) to deal fairly with the medical complaints of contemporary military personnel. The MoD has been criticised by former soldiers, sailors and air crew who believe that their ill-health has been caused during their period of service.

They have waged long and often bitter campaigns against officialdom which, many of them feel, has ignored their claim for compensation and apology. In the 1980s, former servicemen claimed that they had developed cancers and other diseases as a result of exposure to radiation from nuclear weapon tests abroad. In the early 90s, veterans of the Gulf war against Iraq began to experience mysterious illnesses. At first, the MoD denied that the war had anything to do with their diseases. This stonewalling bred accusations of cover-up as well as public cynicism. Labour in 1997 seemed to learn the lesson and promised a new beginning on the issue of Gulf war syndrome. Labour ministers pledged to be more open and give medical help quickly to veterans.

Recently, pressure has been building on another front. More and more of those tested at Porton have been coming forward to complain. Around 250 joined the Porton Down veterans' support group. Their lawyer is now preparing a case against the MoD.

And for the past 18 months, Wiltshire police have been investigating the Porton experiments in a criminal inquiry. They have been looking at allegations that some volunteers were duped into undergoing gas tests by being told they were helping to find a cure for the common cold. The detectives have also been looking at the death of Ronald Maddison, a 20-year-old airman, in a 1953 nerve gas experiment. Following moves by the police, his inquest could soon be re-opened - a remarkable development for a death which occurred nearly 50 years ago.

Defence ministers were forced to consider a health study because of this Wiltshire police inquiry. In November, a senior MoD official wrote and asked the Medical Research Council (MRC) for "urgent assistance and advice on establishing the feasibility of an epidemiological study into the health of service volunteers who attended Porton Down for various trials over a great many years.

"The background to this suggestion is a continuing inquiry by the Wiltshire police into the way in which some of these trials were conducted." The civil servant noted Wiltshire police had uncovered evidence which they believed showed that Porton volunteers were suffering "unusual ill-health" because of their attendance at Porton Down and participation in the trials programme. The MoD appears to accept the nub of the question: "Whilst we are sceptical about police claims, equally there is insufficient scientific evidence on which they could be either confirmed or refuted.

"Our ministers have therefore decided that work should be set in hand to establish whether there is any basis to suggestions that Porton volunteers have encountered premature mortality or unusual ill-health related to their exposures. We believe that a soundly based epidemiological study will be the only way to establish these facts."

The official line is that a study of whether Porton volunteers have died prematurely or suffered cancer is feasible but that it would be much more difficult to look generally at Porton volunteers' health. "Experience from our Gulf war veterans epidemiological projects suggests that very high numbers of people serving in the armed forces many decades ago will be lost to follow up.

"Questions of ethics, consent on the part of the individual to participate in a study, follow up of NHS records, or the alternative of a questionnaire-based study, with the additional uncertainties of response rate, suggest to us that this may not be a worthwhile exercise." But that is to side-step the issue of whether Porton has caused illness which while not life-threatening may have ruined former volunteers' quality of life.

Once they have received the MRC's advice, ministers will decide whether to go ahead with the study. The MoD realises that unless former guinea pigs are seen to be treated in the fairest way possible their sense of grievance may grow.

The Porton experiments happened a long time before the Blair government came to power, so it is surely in ministers' own interest to get the credit for acting to clear up an apparent injustice from the past.

Gassed - British Chemical Warfare Experiments On Humans At Porton Down, by Rob Evans, will be published in paperback by Stratus in March.

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