A group representing veterans who underwent nerve gas tests at a Wiltshire research base has withdrawn its support for a major health study.
The veterans want the government to admit the tests led to ill health
The Porton Down Veterans Support Group (PDVSG) said the study was too limited, and will no longer assist it.
The Medical Research Council (MRC) is overseeing research into cancer and death rates among people who went to Porton Down from the 1940s to 1980s.
The MRC said the scope of the study was limited by the availability of records.
The PDVSG, which has around 550 members, had played an active part in the three-year project, commissioned by the Ministry of Defence (MoD)
Before the start of the main study, a questionnaire was sent to 500 volunteers recruited through the group.
Members of its steering committee also regularly met with researchers working on the study of 20,000 veterans, which began in September 2003.
In a letter to Professor Tom Meade, chair of the Porton Down liaison group, the steering group said: "The parameters set by the MoD...were too narrow from the outset and were, in our opinion, set to produce a relatively predictable result.
It added: "To simply look at the incidences of mortality and cancer does not, in our opinion, come anywhere near addressing the long-term health issues subsequent to exposure to chemical and biological agents."
The committee also said it was disappointed the results of the questionnaire had not been published.
"This does not inspire our confidence," it said.
Committee member Gerry Morris, from Kent, told the BBC there were patterns of health problems suffered by veterans.
This included eye problems, loss of memory at an early age, dizziness and irregular heartbeat, he said.
"We always knew it (the study) was cancer and mortality but we were given the impression, although they didn't say it in concrete terms, that it might be an opportunity to look at other things," Mr Morris said.
The PDVSG hopes to launch its own survey in a bid to force the government to recognise veterans' health has suffered.
An MoD spokesman said it was disappointing the PDVSG had pulled out, but dismissed the claim the ministry wanted "predictable results".
"The parameters of this were set by the MRC not the MoD," he said.
Professor Meade said the MRC was focusing on cancer and mortality because reliable records existed.
He said records of other illnesses were not so reliable and basing a study on veterans' recollections could lead to biased results.
Prof Meade said the MRC hoped to publish the questionnaire's results by the end of the year.
He added he would try to persuade the veterans to come back on board, but stressed the study would continue.
"It's well advanced now and could easily be completed without their involvement," Prof Meade said.
"But I would like to avoid that."