Four million people of working age in the UK are living with untreated high blood pressure.
The estimate is made by the British Heart Foundation (BHF), which is urging people to get themselves checked out.
Its analysis shows that just over a quarter of adults in Britain, around 14.4 million, have the condition.
Four million under the age of 65 have it without being treated and of those, 1.3 million are under 45.
The BHF is urging people to get tested this month, dubbed “May Measurement Month”, as part of a worldwide initiative to make people aware of the risk.
High blood pressure does not usually have any symptoms, meaning that many people are unaware they are living with one of the risk factors most commonly associated with heart attacks and strokes.
© PA The British Heart Foundation is urging people to get checked out for high blood pressure
The condition can be treated easily by a combination of simple lifestyle changes and medication, depending on the individual.
Simon Gillespie, chief executive of the BHF, told Sky News: “I don’t think there’s much public awareness at all.
“You don’t suffer any symptoms of having high blood pressure, it’s only when a critical incident such as a heart attack or a stroke occurs that you may look back and say well possibly it was because of my high blood pressure.
“I think it’s worth remembering that about half of all heart attacks and strokes are actually caused by high blood pressure.
“There is a very high number of people of working age who don’t know their blood pressure and whether or not they have high blood pressure. If they do, and it’s not being treated, they are at significant risk of having a heart attack or a stroke.”
© PA Four million people of working age in the UK are living with untreated high blood pressure
BHF funds have contributed to projects across the country which aim to reach and test people in everyday settings such as train stations, barber shops and football grounds.
The charity says reaching people outside of hospitals and GP surgeries is key to making sure those with undiagnosed high blood pressure understand their risk and are able to get the treatment and advice they need.
Ricky Monahan Brown told Sky News he suffered a stroke in his mid-30s after neglecting to treat his high blood pressure and is backing the call to get tested.
He said: “There was a history of it in my family but I was going around, living my life, and nothing seemed to be out of order so I wasn’t really worrying about that sort of thing.
© Other High blood pressure can be easily treated by lifestyle changes and medication
“I had what’s called a hemorrhagic stroke. What had happened was that my blood pressure was so high that it popped a blood vessel in my brain and when they wheeled me into the emergency room, my blood pressure was at 300 over 200.
“So basically, enough to kill two men and I hadn’t known that was in the post at all. It was very scary, not just for me but for all my loved ones, my friends. I feel terrible about putting them through that.
“The best thing to do is to take the opportunity to check it out when you can. It’s not a risk that’s worth taking, take it from me.”