Scientists in the US say they have demonstrated the existence of the illness known as "Gulf war syndrome".
Many soldiers had numerous injections
The findings are in a report by the influential Research Advisory Committee on Gulf war veterans' illness, leaked to the New York Times.
Committee chief scientist Professor Beatrice Golomb said that exposure to certain substances in the Gulf may have altered some troops' body chemistry.
The study was welcomed by British veterans of the Gulf war.
The secretary of the National Gulf Veterans and Families Benevolent Association, Noel Baker, said the US research was "very explosive".
He added that the Ministry of Defence, which has always denied the existence of a syndrome, would "have to take notice" of it.
"This is very, very senior research. It's not by any private venture or by someone with an axe to grind."
He described the attitude in the US as one of "genuinely wanting to find out if there is a problem.
"In the UK, the MoD doesn't want to find the truth".
The Ministry of Defence declined to comment on the leaked report.
A spokesman said the ministry's position on the syndrome was well-documented, and that there were on-going studies into it.
The ministry argues that there was no single cause of the illnesses reported by veterans from the conflict.
Thousands of veterans of the 1991 war suffer from unexplained poor health.
Servicemen and women from the US, UK, Canada and France who took part in the operation to drive Saddam Hussein's forces from Kuwait have reported one or more symptoms, including memory loss, chronic fatigue and dizziness.
Many continue to suffer from chronic and debilitating illnesses more than a decade since the war.
However, scientists had until now been unable to establish their causes.
The US report said the troops' problems were definitely caused by exposure to toxic chemicals rather than stress or psychiatric illness.
Potential sources include Iraqi nerve gas and drugs given to the troops to protect them from chemical weapons.
"Gulf war veterans really are ill at an elevated degree and several studies bring consistent findings that about 25%-30% of those who were deployed are ill," Professor Golomb told BBC Radio 4's Today programme.
In July, a study funded by the Ministry of Defence and carried out by the London School of Hygiene and Tropical Medicine involved more than 40,000 former soldiers.
It found veterans of the 1991 Gulf War were more likely to report symptoms of ill-health, but similar symptoms were reported by both those who did not serve in the Gulf.