Some 24 per cent of the population do not know they are suffering from high blood pressure, research by the Stroke Association has revealed.
Those with undiagnosed blood pressure are at risk of suffering a stroke, as 150,000 people in UK do each year.
The Stroke Association's tests showed some regional variation, with the rate of undiagnosed blood pressure in the south-west standing at 30 per cent, at 26 per cent in the Home Counties and 18 per cent in the north-east.
The charity has urged people to get their blood pressure tested so that those with the problem can receive medication to deal with it.
Joe Korner, director of the Stroke Association, said: "There is no doubt that blood pressure testing can really save lives.
"Every five mintues someone in the UK has a stroke but over 40 per cent of those strokes could be prevented by the control of high blood pressure.
"High blood pressure can be reduced through medication and controlled by changes in diet and lifestyle," reports the BBC.