If you have been diagnosed with high blood pressure, it could be worth asking the doctor to check again.

The wrong arm position has been linked to far higher readings than if taken the way the NHS recommends – sitting in an upright chair, with it resting on a table or desk.

Other positions, such as resting the arm on the lap or to the side, can substantially overestimate blood pressure readings, researchers found.

Experts suggest it can make a 'huge difference' potentially causing misdiagnoses, highlighting the need for a universal approach to the readings.

Researchers at Johns Hopkins Medicine in the US recruited 133 adults, aged 18 to 80, whose blood pressure was taken during a single visit.

Before readings were taken, all those in the study emptied their bladders, walked for two minutes to replicate somebody coming to the clinic, and then sat down and rested for five minutes with their backs and feet supported.

Each person wore an upper arm blood pressure cuff that fitted their arm and had a series of measurements taken with a digital blood pressure device 30 seconds apart.

They took readings when arms were supported on a desk, arms were supported on a lap, and when arms were unsupported and hung at the patient's side.

Putting an arm on the lap was found to overestimate systolic pressure — the upper number representing the pressure as your heart pushes blood out — by 3.9mmHg.

It also increased diastolic blood pressure — the bottom number measuring the pressure as your heart rests — by 4mmHg.

Meanwhile, an unsupported arm hanging at the side overestimated systolic pressure by 6.5mmHg and diastolic by 4.4 mmHg.

The findings, published in the journal JAMA Internal Medicine, suggest people could end up having high blood pressure readings simply based on the position of their arm, with an arm hanging by the side offering the highest readings.

The NHS advises that normal blood pressure is between 90/60mmHg and 120/80mmHg.

High blood pressure is considered to be 135/85 or higher if the reading was taken at home.

If the reading was taken at a pharmacy, GP surgery or clinic, the figure stands at 140/90mmHg or higher. 

The authors recommend people rest for five minutes before taking an initial reading and then follow it up with a second after several minutes to check it is accurate.

Lead researcher, Dr Tammy Brady, said the different arm positions were often used in clinical practice, perhaps because clinicians were unaware of these differences.

She said: 'This degree of BP error could lead to a substantial number of people being over-diagnosed with hypertension.'

Professor Bryan Williams, chief scientific and medical officer at the British Heart Foundation, added: 'This study reinforces why guidelines are very specific about how to measure blood pressure to ensure readings are accurate. 

'Measuring blood pressure is one of the simplest ways to identify people at risk of heart disease and stroke, but it is important that the measurement is done properly if it is to be used to guide treatment decisions.'