Research has shown that coffee increases production of granulocyte-colony stimulating factor, which stimulates white blood cells and lowers levels of amyloid beta protein in the brain.

Caffeine has also previously been shown to reverse cognitive impairment in aged Alzheimer’s disease mice, while the drinks are also rich in plant chemicals called flavonoids, which boost blood flow to the brain.

Alzheimer’s and other forms of dementia affect 920,000 people in the UK, a figure that will rise to two million by 2050.

However, British researchers said there could be other factors driving the decreased risk and it was too early to know whether tea and coffee really were responsible for better outcomes.

Dr Charlotte Mills, lecturer in nutritional sciences at the University of Reading, said: “The research only shows a link and doesn’t show that the tea or coffee causes the reduced risk of disease. It may be that there are other factors at work.

“However, the finding is consistent with other research showing a link between drinking tea and coffee and other health benefits, such as reduced risk of heart attack or developing Type 2 diabetes.

“One limitation of this work is that the results do not appear to consider milk consumption as a confounding factor.”

Others warned that too much caffeine could raise the risk of dementia and said moderation was key.

David Llewellyn, professor of epidemiology and digital health at the University of Exeter, who has carried out similar research into coffee, said: “Drinking coffee in moderation is associated with a lower risk of dementia, but the highest risk of dementia is associated with heavy coffee consumption.

“We found that drinking more than six cups of coffee a day was associated with 53 per cent increase in dementia risk in comparison with people drinking one or two cups a day.

“If people are already drink tea or coffee in moderation then increasing their consumption further could potentially increase their risk of having a stroke or developing dementia. The potential harm linked with heavy consumption is particularly clear for coffee.

“In theory, people who don’t drink tea or coffee at all may benefit from starting to drink in moderation, and heavy consumers of tea and coffee may benefit from drinking less.

The research was published in the journal PLOS Medicine.