People who have blood pressure readings even slightly higher than normal may have an increased risk of stroke, new research suggests.

Scientists in the US have come to the conclusion after analysing data on 518,520 adults from 12 studies looking at blood pressure and stroke risk. They found that people with "prehypertension" - slightly raised blood pressure - were 50% more likely to suffer a stroke than those with normal blood pressure levels.

Prehypertension is defined by a systolic (top number) reading of 120 to 139 millimetres of mercury (mmHg) and a diastolic (bottom number) of 80 to 89 mmHg. "Normal" blood pressure is said to be at or below 120 (systolic) over 80 (diastolic). "High" blood pressure has consistent readings of 140 over 90 or more.

People under the age of 65 with prehypertension had an almost 80% increased risk of stroke when compared with normal blood pressure individuals, according to the research.

"These people may immediately benefit from blood pressure-lowering methods, such as reducing their salt intake and weight, to help reduce their risk of stroke," said study leader Dr Bruce Ovbiagele, from the University of California at San Diego.

The research, published online in the journal Neurology, took account of influencing risk factors such as age, gender, diabetes, obesity, cholesterol and smoking.

Dr Sharlin Ahmed, from The Stroke Association, said: "High blood pressure is the single biggest risk factor for stroke and it affects around 16 million people in the UK. However, 40% of all strokes could be prevented if people took steps to control their blood pressure.

"Even a small increase in blood pressure could lead to high blood pressure in the future, so it's very important to have regular checks. Making a few simple lifestyle changes, such as eating a healthy diet low in salt, giving up smoking, and exercising regularly, can help to keep your blood pressure under control and can reduce your risk of stroke." 

Why not keep a check on your blood pressure with our World Health Organisation (WHO) claasification arm blood pressure monitor.