The study - the largest ever of its kind and the first major research to look at people before they develop Alzheimer's - followed 6,870 men and women aged 55 and over.

None of the people had Alzheimer's, the most common form of dementia, before the study by the Erasmus Medical School in Rotterdam began.

Dementia

Over a two-year period, any who developed signs of dementia were assessed and, where possible, given a brain scan.

A total of 146 people developed dementia during the course of the study, with 105 being diagnosed as having Alzheimer's.

People who smoked were found to be 2.3 times more likely to develop Alzheimer's than those who had never smoked.

They were also more likely to get Alzheimer's at a younger age.

Protective

However, the researchers found that smoking does not increase the risk of developing Alzheimer's for people with a gene linked to the disease.

Indeed, they believe smoking may offer protection against the disease for people with the gene - APOE epsilon 4.

"It seems that if you have the gene, you're better off if you smoke," said Dr Monique Breteler, one of the senior researchers.

Beneficial

Previous research said smoking protected from dementia

Previous studies of the links between smoking and Alzheimer's have suggested smoking could have a beneficial effect on the disease. But their findings have been inconclusive.

The Dutch researchers said this could be because smoking altered the chemistry of the brain and defused some of the effects of Alzheimer's.

Another reason could be the fact that many smokers do not live long enough to develop the disease, which particularly affects the very elderly.

Powerful

Dr Anthony Mann, an expert in the treatment of elderly people from the Institute of Psychiatry in London, said the new research presented "powerful" evidence.

"They are the first to do a prospective study and it's the largest to show a positive link," he said. "It's the best we've had. It takes forward the notion that things that put you at risk for vascular disease, put you at risk for dementia in general."