A quarter of people with diabetes in England have not had the condition diagnosed, a study suggests.
More than 1.7m people in England have diabetes
Among women over 40 who had diabetes, 45% had not had it diagnosed, the Healthcare Commission reported.
The NHS watchdog carried out an audit in 2003-4 of 250,000 patients in the country, covering 1,700 GP practices and 47 hospital trusts.
More than 1.7m people in England have diabetes - 75% of whom are type 2, which is linked to obesity.
Campaigners said the NHS must make sure people are identified before long-term complications took effect.
The NHS watchdog carried out an audit of 250,000 patients in the country, covering 1,700 GP practices and 47 hospital trusts in 2003-4.
The study concluded that 77% of diabetes patients were diagnosed after comparing GP records of diabetes patients to epidemiological estimates of the numbers with the condition, but the figure dropped to 55% for women over 40.
It was suggested that the reason for the lack of diagnosis among older women was because the only systematic diabetes screening was for heart disease patients, a greater proportion of whom are men.
The watchdog also found fewer than 50% of diagnosed people were receiving eye tests - putting them at risk of blindness, one of the long-term complications of diabetes.
Some 56% of diabetes patients were not managing their glucose levels within the official guidelines, the report said.
The watchdog recommended that yearly checks on glucose, cholesterol and blood pressure levels be carried out for most patients.
Healthcare Commission chief executive Anna Walker said she wanted the NHS to make improvements and local health bosses would be judged on it in the future.
"We expect to see changes as a result of this work and we will be using participation in the audit as part of our annual performance ratings from primary care trusts.
"It is very important that patients and the public are informed of and understand the benefits of treatment and follow-up care for diabetes, as well as the risks of long-term complications that will result without diagnosis or treatment."
Douglas Smallwood, chief executive of Diabetes UK, said it was important that NHS services stopped letting undiagnosed patients slip through net and the public became more aware of the condition.
He added: "The longer people are left undiagnosed or are not managing their diabetes, the greater the risk of long-term complications such as heart and kidney disease, blindness and amputations."
Dr Sue Roberts, the Department of Health's national clinical director for diabetes, said "improvements will be needed".
She added: "This information is already telling us a lot about the level and extent of diabetes care in England."