The implication of a Congressional report that large numbers of civilians and troops in Iraq and neighbouring countries could have been exposed will galvanise the controversy over illnesses suffered by more than 5,000 British veterans since 1991 that have been linked to their service in the Gulf.

The report indicates that possible chemical contamination of troops could have been much more widespread than suggested by previous official government estimates, based on US research for the Pentagon and CIA.

Lord Morris, the Labour peer who has led the campaign on Gulf war illnesses, yesterday demanded answers from the government, saying it appeared the entire British deployment of more than 50,000 troops could have been at risk.

The MoD used the US defence department models to estimate that 9,000 British troops were within the chemical plume that might have been released from the destruction of chemical agents at Khamisaya, in southern Iraq, in March 1991. This figure was revealed in 1999. Previously, the government said no British units would have been affected, although one Briton might have been under a plume.

More than 5,000 British veterans have reported illnesses they believe related to the Gulf war or the inoculations they received before deployment and more than 600 have died. The government has refused to accept any suggestion that there is a "syndrome" but points to its £8.5m research programme to prove its commitment to finding answers.

The government's current position is that the possible level of nerve agent exposure from Khamisaya would have had "no detectable effect" on human health, and the Pentagon still insists the information was the best available and any researcher would know limitations of the data. The CIA also agreed with the report.

But the general accounting office (GAO), the investigative arm of Congress, last week said the assumptions used by the Pentagon were based on incomplete and uncertain data and that postwar testing to replicate the size of the plume "did not realistically simulate the actual conditions of bombings or demolitions". It also beleived there was substantial empiracal evidence that low level exposure to agents, inclusing sarin, could cause symptoms similar to those suffered by Gulf war veterans.

The Pentagon, including the bombing of other sites in Iraq, estimated that nearly 102,000 US troops were potentially exposed. But the GAO concluded that, given the significant methodological flaws, neither the Pentagon nor the MoD could know which troops were and which troops were not exposed. And research seeking to compare later health of soldiers who had been exposed and those who had not was also therefore flawed.  In addition, many suffering illnesses might .  And research seeking to compare later health of soldiers who been exposed and those who had not was also therefore flawed.  In addition, many suffering illnesses that did not lead to hospital treatment. 

Another model had suggested the height and dispersal of plumes could be far higher.  Sushil Sharma, assistant director of applied research and methods at the GOA, said: "According to the projections made by a validated model that was ignored by [defence department], anyone, including civilians, who was in the theatre of war could have been potentially exposed.  Not all troops have shown symptoms but many have.

Lord Morris, an honorary member of a US congressional sub-committee investigating undiagnosed illnesses, said: "This is a profoundly significant report not only for US veterans but for ours as well. It started as one, then it was 9,000, and the implication of this is we could have over 50,000"

He has tabled a parliamentary question to ministers on the issue. "This strengthens the case for a full and independent inquiry into Gulf war-related illnesses."

Terry English, director of welfare for the Royal British Legion, said: "I think veterans will be convinced this confirms all their beliefs. I cannot see the government accepting it lying down but it is good to get in on the record."

The MoD has promised to make a further assessment of the US modelling and will respond to the GAO report in September. It has refused demands for a public inquiry into the illnesses although it has not ruled one out.

A former solider, Alex Izett, is on hunger strike at his home in Germany, in an effort to force an inquiry. He alleges he was made ill by the vaccination programme.

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