An estimated 25,000 Kingston adults are walking around with high blood pressure - and nearly half of them do not know it.
Public health bosses say 20 per cent of over-18s may have the condition, which can lead to heart attacks and strokes.
But GPs report just 11.5 per cent of their patients have it diagnosed.
Kingston Council's director of public health, Dr Jonathan Hildebrand, said: "There are people around who are not being diagnosed [with high blood pressure].
"It's a risk for a number of cardiovascular events like heart attacks and strokes."
Kingston's 20 per cent estimate, which could mean more than 25,000 people, is significantly lower than the 32 per cent figure health bosses believe applies nationally, Dr Hildebrand said.
He said: "It shouldn't be alarming.
"Diet and exercise would be the prime lifestyle interventions.
"For a number of people, that will control their blood pressure."
Kingston Council offers free health checks every five years for people aged 40-74 who have not already been diagnosed with a vascular condition.
The checks, available at GP surgeries across the borough, highlight a raft of issues including family health history, weight and cholesterol levels.
Dr Hildebrand added: "Having the health checks is a sensible idea."
Kingston Council's cabinet member for health, Councillor Julie Pickering, said: "I think that a lot of people have underlying conditions that they don't know about.
"I would like to say that we can get that figure down.
"Anything that people can do to support their own health is the way we have to go.
"Have yourself checked out every so often."
Liberal Democrat health spokesman Coun Yogan Yoganathan said: "I think we need to make sure that information is passed on to people because they're not aware.
"It will save money for the NHS."
The Surrey Comet reported earlier this year that more than half of adults in Kingston - 55.1 per cent - are overweight or obese.
Problems associated with obesity cost the NHS more than £5bn each year.
In 2012/13, less than 11 per cent of eligible residents took up the offer of a free health check.
To find out more about getting a health check, call 0800 028 8694 or email firstname.lastname@example.org.