The dictator suffered the disease, and the mental inflexibility associated with it could have been what led to his slow response to the D-Day landings in Normandy in 1944, researchers said at the International Congress on Parkinson's Disease in Vancouver.

Dr Tom Hutton, a neurologist who co-authored the study, said Hitler was suffering physical and mental symptoms of the disease, but his aides kept it secret.

He said that by the time of the Normandy landings, Hitler had suffered the disease for 10 years and would have had problems processing conflicting information - hence his initial refusal to allow Panzer divisions to move to the site of the invasion.

Hitler is said to have been convinced that the Allies would launch their attack at Calais.

Debilitating disease

Parkinson's disease is a degenerative disease of the nervous system that generally affects both men and women who are more than 40 years old. However, 10% of sufferers are believed to be under 40.

Hitler was slow to react to the Normandy landings

The disease develops slowly and is associated with trembling of the arms and legs, stiffness and rigidity of the muscles and slowness of movement.

A third of Parkinson's suffers also develop senile dementia.

In many cases, sufferers eventually die from secondary complications such as pneumonia, urinary tract infection, pressure sores, septicaemia and stroke.

It is thought to be caused by the death of nerve cells that would normally produce a chemical - dopamine - that carries messages around the nervous system.

"Hitler's slowness to counterattack at Normandy may have been secondary to mental inflexibility and difficulty in shifting concepts due to Parkinsonism," Dr Hutton's discussion paper said.