Using targeted screening to check for adult-onset diabetes could reveal tens of thousands of undiagnosed cases each year, research has suggested.
Just under three per cent of the over-40s screened had Type 2 diabetes in the St Mary's Hospital London study.
Doctors there estimated around 500 patients who did not know they had diabetes could be detected using this method each year.
The research is published in the journal Diabetic Medicine.
There are currently up to one million people in the UK who have diabetes but do not know it.
As Type 2 diabetes can go undiagnosed for up to 12 years, around half the people with the condition already have evidence of complications at diagnosis.
Groups at high risk of the condition include white people aged over 40 and people from black and minority ethnic groups aged over 25 who are overweight or who have a family history of diabetes.
Blood glucose levels
Five hundred patients, all over 40, attending the A&E unit at St Mary's were randomly selected to take part in the study.
All completed a questionnaire regarding risk factors for diabetes.
Seventy-three people were found to have been diagnosed with diabetes.
But 13 - 2.6% - were found to have Type 2 diabetes. These people had previously been unaware of their condition.
Based on their findings, the researchers estimate over 500 new cases of Type 2 diabetes could be diagnosed at the hospital each year.
Screening also identified people (4.6%) who did not have diabetes, but have higher than normal blood glucose levels which puts them at increased risk of developing the condition.
Dr Peter George, one of the researchers at St Mary's Hospital, said: "Identifying those individuals who are unaware of their diabetes is currently a major public health issue."
And Dr Jonathan Valabhji, a consultant diabetologist at St Mary's added, "Opportunistic screening for diabetes in hospital A&E departments, particularly of those in high risk groups, could also make an important contribution."
Douglas Smallwood, chief executive at Diabetes UK said: "Screening the general population would be a waste of resources.
"However, a targeted screening programme of people who are at high risk of coronary heart disease, are overweight or have a family history of heart disease or diabetes, should be considered.
"We also recommend that GPs test their patients who fall into these risk categories."
He added: "The sooner people are diagnosed, the sooner people can start taking control of diabetes and help reduce their risk of developing long term complications.
"The time for delays is over. The NHS needs a programme for early identification targeting those at increased risk of diabetes."