The team looked at lifelong data from 500 participants, who were all born in the same week in 1946, in order to assess démence risk. They also looked at pression artérielle measurements taken throughout the participants' lives.

Brain scans looked for levels of a key Alzheimer's protein called amyloid. Brain size and presence of blood vessel damage in the brain were also taken into account.

An increase in blood pressure between the ages of 36 and 43 was linked to a decreased brain volume. Rises in blood pressure between the ages of 43 and 53 were also linked to blood vessel damage or 'mini-strokes' in people over 70. However, blood pressure was not associated with the amount of amyloid protein in the brain.

Lead author Professor Jonathan M Schott, University College London, said: "We now know that damage caused by high blood pressure is unlikely to be driven through the hallmark Alzheimer's protein amyloid, but through changes in blood vessels and the brain's architecture. The findings show that blood pressure monitoring and interventions aimed at maximising brain health later in life needs to be targeted at least by early midlife.

"NHS health checks are currently offered from the age of 40, and the uptake is, at most, 50%. Our data suggests that blood pressure should be measured much earlier," he told the BBC.

Dr Carol Routledge, director of research at Alzheimer's Research UK, said: "High blood pressure in midlife is one of the strongest lifestyle risk factors for dementia, and one that is in our control to easily monitor and manage. Research is already suggesting that more aggressive treatment of high blood pressure in recent years could be improving the brain health of today's older generations.

The research is published in The Lancet Neurology.