Nearly one in four people with high blood pressure are at risk of a stroke because the condition has not been diagnosed, a charity says.
Strokes are caused by bleeds or clots in the brain
The Stroke Association examined blood pressure readings for 19,318 people across the UK.
A quarter with high blood pressure did not know they were affected - a figure which rose to 30% in the south west.
Experts urged people to get their blood pressure tested to avoid becoming a victim of the "silent killer".
Each year, an estimated 150,000 people have a stroke in the UK - largely caused by disruption of blood flow to the brain due to a blood clot.
About a third of those are likely to die within the first 10 days, about a third are likely to make a recovery within one month and about a third are likely to be left disabled and needing rehabilitation.
But many strokes could be prevented if people lowered their blood pressure by stopping smoking, eating less salt and fat, doing exercise and watching their weight.
High blood pressure also increases the risk of a heart attack.
Blood pressure tests carried out by the Stroke Association showed 26% of people in East Anglia and the Home Counties with high blood pressure had not realised they had the condition.
Those in the north east were most aware with 18% found to have undiagnosed high blood pressure.
The Stroke Association together with Rotary International Great Britain and Ireland (RIBI) are planning a series of blood pressure testing days across England.
Joe Korner, director of the Stroke Association, said: "There is no doubt that blood pressure testing really can save lives.
"Every five minutes someone in the UK has a stroke but over 40% of those strokes could be prevented by the control of high blood pressure.
"We also know that many thousands of people are completely unaware of their own blood pressure levels.
"Many of those people have no idea that they have high blood pressure and that their risk of having a stroke is massively increased.
"High blood pressure can be reduced through medication and controlled by changes in diet and lifestyle.
Andrew Lansley, shadow health secretary and chair of the All Party Parliamentary Group on Stroke, said: "People lead busy lives but getting your blood pressure checked, either by your GP or at a Stroke Association/RIBI testing day near you is quick, simple and could change your life for the better.
"An estimated 150,000 people have a stroke each year, with a quarter happening to those under 65, so monitoring blood pressure is absolutely vital."
Sue Massey, spokesperson for the Blood Pressure Association, said: "One in three people in the UK have high blood pressure, the major cause of death and disability through stroke and heart attack.
"The good news is that high blood pressure can be treated and often prevented, but the only way to find out if you have it is by having a check."