Almost 800,000 diabetics are risking blindness, heart disease and kidney failure because their blood sugar levels are not under control, figures have showed.
An analysis of more than two million diabetics aged over 17 in the UK found 34% (797,067) were not hitting their recommended blood glucose levels.
Keeping blood glucose levels to between 6.5% and 7.5% cuts the risk of long-term complications from the disease.
The research was released by Diabetes UK to mark World Diabetes Day.
Douglas Smallwood, chief executive of the charity, said: "It is absolutely appalling that one in three people with diabetes are struggling to achieve their blood glucose targets.
"Diabetes complications are estimated to cost the NHS around £1 million an hour and can have a devastating effect on people's quality of life. It is vital that all of the 2.6 million people with diabetes are armed with the knowledge and confidence to manage their diabetes effectively.
"Diabetes UK is calling for better support for people with diabetes on World Diabetes Day to manage their diabetes. People with diabetes make decisions about their diabetes management every day and want to maintain an independent, healthy and active life. If they need support to achieve this, it should be widely available."
Mr Smallwood said there was patchy progress on implementing Government plans for everyone with a long-term condition to have a care plan by 2010.
The figures come as new research from the Juvenile Diabetes Research Foundation (JDRF) found that few people know the difference between Type 1 and Type 2 diabetes.
Type 1 diabetes is an autoimmune condition that is frequently diagnosed in childhood or adolescence. Patients are dependent on insulin to survive. Type 2 diabetes is linked to unhealthy lifestyles and obesity and patients may manage their condition through diet, tablets or injectable insulin. Both types can cause complications, including the need for amputation due to poor circulation, kidney failure, heart disease and blindness.
