More people may have been exposed to chemical warfare agents during the 1990 Gulf War than previously thought, a report says.
Some 700,000 troops served in the 1990 Gulf War
The US government revealed in 1996 that some people may have been exposed to chemicals when troops destroyed a stockpile of agents in southern Iraq.
As well as Iraqis, officials said over 100,000 troops, including 9,000 Britons, may have been affected.
But the US General Accounting Office says the figures could be much higher.
The GAO is the investigative arm of the US Congress. It has carried out an investigation into how the US and UK governments came up with their figures on how many troops may been exposed to these agents.
The figures are largely based on how many troops were in the direct path of the plume of smoke that was produced when these agents were destroyed at Khamisiyah, in southern Iraq in 1991.
But in a report, the GAO says the figures, which were drawn up by the US Department of Defense, "cannot be adequately supported".
It says the plume of smoke travelled further than officials have suggested.
"The plume heights used in the modelling were underestimated and so were the hazard area," it says.
The report dismisses the estimates on how many troops may have been exposed to the agents as "uncertain, incomplete and non-validated".
The report raises serious questions about UK government claims that only 9,000 British troops were in the direct path of the plume of smoke.
"Since the Ministry of Defence relied exclusively on Department of Defense modelling and since we found that Department of Defense could not know who was and who was not exposed, the MOD cannot know the extent of British troops' exposure."
The Ministry of Defence said it was considering the findings of the report.
"We will publish a paper in September reviewing the results of the US modelling and our response to the GAO report," a spokeswoman told BBC News Online.
More than 5,000 British veterans who served in the Gulf War have reported illnesses, which they believe may have been caused by vaccines or exposure to chemicals.
The government has so far refused to acknowledge that Gulf War syndrome exists.
The UK's National Gulf War Veterans and Families Benevolent Association renewed their calls for a public inquiry into gulf war syndrome in light of the GAO findings.
"It is possible that the majority of troops deployed in the Gulf in 1991 may have been exposed to these chemicals," said Shaun Rusling, its vice chairman.
"We need a full public inquiry. The Ministry of Defence has known about this for years."