Heavy smoking in mid-life more than doubles the risk of developing Alzheimer's disease, a study has shown.

Smoking more than 40 cigarettes a day also greatly increased rates of another common form of dementia.

Researchers in the US followed the progress of more than 21,000 middle-aged men and women for an average of 23 years.

Those who consumed more than two packets of 20 cigarettes a day had a 157% higher chance of suffering Alzheimer's than non-smokers.

They also had a 172% increased risk of vascular dementia over the follow-up period.

Vascular dementia is the most common form of the disease after Alzheimer's and is linked to poor blood supply to the brain.

Lead researcher Dr Rachel Whitmer, from the Kaiser Permanente research institution in Oakland, California, said: "This study shows that the brain is not immune to the long-term consequences of heavy smoking.

"We know smoking compromises the vascular system by affecting blood pressure and elevates blood-clotting factors, and we know vascular health plays a role in risk of Alzheimer's disease."

Participants in the study were enrolled into a dementia survey between 1978 and 1985 when they were 50 to 60 years old.

A total of 1,136 were eventually diagnosed with Alzheimer's and 416 with vascular dementia.

Heavy smoking in middle age was associated with a higher risk of dementia overall, as well as a greater chance of developing each of the sub-types.

Former smokers, or those who smoked less than half a pack a day, did not appear to face an increased risk.

The findings, published today in the journal Archives of Internal Medicine, were not affected by race or gender.

Smoking is known to contribute to inflammation and oxidative stress, both of which are believed to be important in the development of Alzheimer's disease.

Its effect on blood vessels may account for the increased risk of vascular dementia, said the researchers.

The authors wrote: "To our knowledge, this is the first study evaluating the amount of mid-life smoking on long-term risk of dementia and dementia sub-types in a large multi-ethnic cohort.

"Our study suggests that heavy smoking in middle age increases the risk of both Alzheimer's disease and vascular dementia for men and women across different race groups.

"The large detrimental impact that smoking already has on public health has the potential to become even greater as the population worldwide ages and dementia prevalence increases."

Around 750,000 people in the UK suffer from dementia, most of whom are afflicted by Alzheimer's.

By 2021 their numbers are expected to swell to more than 940,000.