Experts have begun a review of blood pressure monitors after concerns were raised about their accuracy.
Blood pressure monitors will be evaluated by the group
Patients can now buy monitors over-the-counter in chemists, from the Internet, or from adverts in magazine supplements.
But there have been reports that some monitors "under-record" blood pressure.
This could have a significant impact on patients, as the higher their blood pressure, the more serious the risks for their health.
The expert group, which has been set up by England's Chief Medical Officer Sir Liam Donaldson to look at the use and accuracy of different types of
blood pressure monitors, met on Tuesday for the first time.
Automated monitors are the most popular alternative to mercury monitors.
But concerns about their accuracy have been reported, particularly in the management of conditions such accelerated hypertension (a severe form of high blood pressure), pre-eclampsia and cardiac arrhythmias (heart rhythm disorders).
The expert group will provide guidance about the accuracy of various types of monitors available to doctors, who will then be able to pass it on to patients.
Professor Andrew Shennan, a leading expert in hypertension and director of the Maternal and Foetal Research Unit at St. Thomas's Hospital, said: "Measuring blood pressure is an important part of diagnosing and treating many medical conditions.
"With mercury blood pressure monitors being phased out and concerns over the accuracy of automated monitors, doctors are in need of practical advice on the different types of monitor available and when they should be used."
Professor John Potter, chairman of the British Hypertension Society's blood pressure monitoring committee, told BBC News Online: "The monitors we are most concerned about are the ones people can buy over-the-counter, from the colour supplements or on the Internet.
"Most of those used by doctors have been validated. Most of those sold over-the-counter have not."
He added members of the public should not simply buy the most expensive monitor they could find. "Just because you spend a lot of money doesn't mean it's accurate"
Professor Potter, who is a member of the expert group, said anyone who did want to buy a blood pressure monitor should check the British Hypertension Society's website to see which models it recommended.