Dr Saleh Naser and researchers at the University of Central Florida in Orlando believe it is due to a bacterium called MAP which is found in cattle, sheep and goats suffering from an illness called Paratuberculosis or Johne's disease.

"We discovered the bacteria in the blood of Crohn's patients. This is the first time anyone has done that," Naser said in an interview.

"There is strong evidence that this bacteria may be responsible for Crohn's disease."

Previous studies have concentrated on looking for MAP in the tissue of Crohn's patients and the outcome has been mixed, according to Naser.

MAP was found in the blood of patients with Crohn's disease but not in healthy people.

"The blood is a sterile environment so the presence of this bacteria in the blood indicates this disease might be systemic, which means it may start in the intestine and ultimately it may infiltrate into other organs," he said.

Abdominal pain, diarrhoea, rectal bleeding, weight loss and fever are the most common symptoms of the illness.

Naser, who reported the finding in The Lancet medical journal, believes people can be exposed to the bacteria but they do not develop the illness unless they have a genetic susceptibility to it.

In a commentary in the journal, Professor Warwick Selby of the Royal Prince Alfred Hospital in Newtown, Australia, said although the research may fall short of proving that MAP is one of the causes of the illness, it raises many important questions.

"The findings now need to be replicated in other laboratories. Whatever one's view, MAP cannot continue to be ignored in Crohn's disease," he said.