Up to one-quarter of the six million people being treated for high blood pressure in the UK may have been misdiagnosed.
The standard way of measuring blood pressure is inaccurate and should be replaced by 24-hour monitoring, using a device worn on the waist, experts advise.
The recommendation, by a panel of the National Institute for Clinical Excellence (Nice), is expected to be adopted around the world - the first change to the way blood pressure is measured for more than a century.
Traditionally, blood pressure is taken by a cuff attached to the patient's upper arm and pumped up before being released slowly to measure the pressure at which blood starts to flow again.
Many patients suffer from "white coat syndrome" - their blood pressure rises because of anxiety triggered by a visit to the surgery. A more accurate measure can be obtained by monitoring blood pressure for 24 hours using a device attached to the waist that pumps up the cuff worn around the arm every half hour and takes a reading, all of which are then averaged out.