Almost one in 70 people are living with undiagnosed diabetes and are running the risk of "devastating" complications, the UK's leading diabetes charity said.
There are an estimated 850,000 cases of undiagnosed Type 2 diabetes in Britain, meaning about one in 74 people has it but does not know, according to Diabetes UK.
That equates to at least one passenger on a full double-decker bus or about 1,000 people in a full Wembley Stadium having undiagnosed diabetes, the charity said.
Sufferers are missing vital health checks and those who leave the condition untreated run a greater risk of complications such as amputation, blindness, kidney failure and stroke.
Risk factors for Type 2 diabetes include being overweight, having a large waist and being physically inactive. Older people and people from a black or South Asian background are also at higher risk, as are people with a family history of the condition.
To mark the start of Diabetes Week, the charity and healthcare specialists Bupa will today attempt to break the world record for the number of waists measured in eight hours.
The record attempt will launch a series of roadshows around the country where teams will refer people who are at high risk to their GP.
This will mean that people who have the condition can be diagnosed and those who do not have it but are at high risk can be given the advice and support to help prevent it.
Barbara Young, chief executive of Diabetes UK, said: "When you consider the potentially devastating health consequences of Type 2 diabetes, it is shocking that so many people have the condition and do not know it. These figures show that every time we walk down our local high street, we are likely to be walking past people who have undiagnosed Type 2 diabetes.
"This is a real concern because it is only by getting the condition diagnosed early that people can start getting the treatment they need to prevent serious health complications, including blindness, amputation, kidney failure and stroke. Getting these people diagnosed is a race against time and unfortunately it is a race we are all too often losing.
"We are also encouraging people to talk to their friends and family about Type 2 diabetes.
"Making them aware that someone can have it for a number of years without realising it could be the vital first step towards someone being diagnosed and getting the healthcare that can give them the best chance of a long and healthy life."
Diabetes is a condition in which the amount of glucose in the blood is too high because the body cannot use it properly.
Type 2 diabetes develops when the body can produce some insulin, which breaks down glucose, but not enough, or when the insulin that is produced does not work properly.
Health Minister Simon Burns said: "We are working with the NHS to improve diagnosis rates for diabetes. Our NHS Health Check programme is already playing a growing role in early detection and has the potential to prevent over 4,000 people a year from developing diabetes and detect at least 20,000 cases of diabetes and kidney disease earlier.
"By getting an earlier diagnosis we can make sure that their condition is better managed which can lead to an improved long-term quality of life."
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