The recommended way to diagnose high blood pressure is to change for the first time in more than a century, potentially identifying a quarter of patients who could have been misdiagnosed.

Guidelines published by the National Institute for Health and Clinical Excellence recommend that patients should be monitored for 24 hours to determine if they have high blood pressure.

This process, ambulatory blood pressure monitoring, involves wearing a cuff and a box on a belt for a day as the patient goes about their daily life, to give a series of readings under normal conditions.

Currently patients have a number of appointments to have their blood pressure checked, and it is estimated that 25 per cent suffer from “white-coat hypertension” – nervousness when seeing medical staff – which will cause a high reading.

Cathy Ross, senior cardiac nurse at the British Heart Found-ation, said: “The number of people with high blood pressure in the UK is staggering. It’s a major risk factor for heart disease and strokes, so it’s crucial we do all we can to get people diagnosed and properly treated as soon as possible.

“This new guidance will refine the way we test and treat people for hypertension, particularly by helping to identify people suffering from ‘white-coat hypertension’.”

Chairman of the guideline development group Bryan Williams, professor, said any patients who have been misdiagnosed will be identified during the standard review process.

The guideline panel expect it will take around a year to roll out and that the cost of the machines, currently around £1,000 each, could halve.

Under the new advice, it is also recommended that people over 80 are treated for high blood pressure.

In the past there has been debate over whether this will cause more harm than good. Prof. Williams said: “The message is that you’re never too old to start treatment for hypertension, and for the most part that treatment will provide benefits.”

The guidelines have also changed the recommended diuretic used to treat hypertension from a thiazide diuretic to one such as chlortalidone or indapamide, as studies have shown they are more effective. The advice favours prescribing a calcium channel blocker instead of a diuretic, again because evid-ence suggests this has greater efficacy.