Analysis, based on data from the National Diabetes Audit, shows that just 19.9% of people with either type 1 or type 2 diabetes in England meet the recommended targets for blood glucose, blood pressure and cholesterol. In Wales, this figure is just 18.5%.
Out of those in England with type 1 diabetes, only 11.4% are meeting the recommended levels.
Barbara Young, chief executive of Diabetes UK, said: "When you consider that there are now three million people diagnosed with diabetes and this number is rising quickly, the fact that so many of them do not have good control over their diabetes means that unless something changes we face a public health disaster.
Baroness Young accused the healthcare system of being "all too often not good enough", and said that people with diabetes needed self-management support programmes, along with ongoing medical support and education if they were to avoid complications and reduce the risk of early death.
Diabetes UK is calling on the government to increase the number of people with diabetes getting the nine annual health checks recommended by the National Institute for Health and Clinical Excellence, and to hold poor-performing areas of England and Wales to account.
"It is time to break the cycle of poor diabetes management and poor health outcomes," said Baroness Young.
"By investing in the healthcare and ongoing structured education to enable people with diabetes to manage their condition, we can ensure they have the best possible chance of a long and healthy life," she added.
A charity has called for more specialist nurses to be trained in supporting patients use insulin pumps after evidence suggested they are more effective at controlling blood glucose than insulin injections in children with type 1 diabetes and cause fewer complications.
People with diabetes have been warned to check if they have a faulty blood glucose meter at home that could give a false reading and delay treatment.
Young adults with Type 1 diabetes are much more likely to die than their peers because they are not getting the healthcare they need to manage their condition, Diabetes UK has warned.