British veterans of the first Gulf war and their supporters yesterday demanded that the Ministry of Defence accept that many were ill because of their service 13 years ago, following further evidence that US medical advisers were prepared to do so.

A panel commissioned by the US government has concluded there is a "probable link" between exposure to toxic chemicals and illnesses in many troops who served in the Gulf in 1991.

Its leaked draft report cited by the New York Times at the weekend suggests that "a substantial proportion of Gulf war veterans are ill with multisymptom conditions not explained by wartime stress or psychiatric illness".

Possible sources include sarin, a nerve gas released from weapons depots, as well as two measures taken to protect troops, tablets to protect against nerve agents and pesticides, both liberally used during the conflict. There is a growing feeling that a combination of these and other factors might have led to more serious consequences for the health of veterans than was realised both in the US and here, but the ground began shifting across the Atlantic at least two years ago.

The leak, with the presidential election looming, could yet have political impact here, feeding into the continuing fallout over the reasons the government gave for entering the second Iraq conflict. The US report apparently suggests that understanding the illnesses has important implications for future deployments abroad and security at home.

The research advisory committee on Gulf war veterans' illnesses, set up by the US department of veterans' affairs, seems impressed by scientific reports of recent years suggesting that low-level toxins can have a serious, if delayed, effect on health. These had already begun to change minds in military, political and scientific circles in the US, ending a previous consensus that many problems were stress-related, and some have been presented to Lord Lloyd's independent inquiry into veterans' illnesses here.

But the MoD has refused to take part in the inquiry and still hold the view that even if many more veterans report ill-health than other groups, "there is insufficient evidence to enable this ill-health to characterised as a unique illness or syndrome". It refused to comment on the leaked report.

More than 6,000 British veterans and 100,000 in the US have complained of ill-health due to service in the Gulf. The Lloyd inquiry is expected to report before the middle of next month and Lord Morris of Manchester, a prime mover in its establishment, said the leak was "a major development in unravelling the truth about Gulf war illnesses".

Malcolm Hooper, scientific adviser to veterans, said: "This is very, very important. It is saying what some of us here in this country have been saying for a long time and it is high time the Ministry of Defence woke up to it."

The US attitude had in 1997 been characterised in a congressional report as "a tin ear, a cold heart and a closed mind" but there had been "a complete sea change in the United States". Prof Hooper added: "Here it has not changed but it won't wash any longer."

He suggested that Britain could do more to research into possible damage from the battery of vaccinations given to troops before the conflict. It should also accept that veterans had a higher level of motor neurone disease, a factor in changing US minds.

Charles Plumridge, of the National Gulf Veterans and Families Association, said: "This is the scientific evidence the MoD has asked for. It comes from a group with so much clout no one should try to argue with it."

Earlier this summer, US congressional investigators said all troops and civilians in the region might have been exposed to low levels of chemical agents following bombing and post-war demolition of chemical plants and munitions centres. The MoD has not responded.