Thousands of cases of dementia could be prevented by increasing levels of lithium in tap water, a major new study has suggested.

Scientists at the University of Copenhagen compared dementia rates to the natural quantities of lithium in water for more than 800,000 people in Denmark, from areas occupied by nearly half of the population.

They found that in places where lithium was highest, the dementia rate fell by 17 per cent compared to those with the lowest levels.

Although researchers warn that the link could be due to other environmental factors, they say that it is worth investigating whether adding lithium to tap water could be a cheap way of protecting large numbers of people from dementia.

Lithium is a metallic element which is found in varying quantities in water in Britain, from around one migrograms per litre to around 21 micrograms per litre. The researchers found benefits after 15 micrograms per litre.