In England, there are around two million people diagnosed with diabetes. Another million have the condition but do not yet know it.
Some of the errors are caused by mistakes made when entering information, but some are down to a lack of understanding among doctors or other staff, the report said.
It said “the most widespread misunderstanding” among health professionals was changing somebody’s diagnosis from Type 2 to Type 1 when they go on to insulin.
“This, potentially, could have a considerable impact on patient care as the guidelines for insulin use in Type 2 are very different from those in Type 1.”
But Dr Rowan Hillson, National Clinical Director for Diabetes, said: “In a few complex cases people may have been misdiagnosed with diabetes or the type of diabetes misclassified, but this shouldn’t significantly affect their treatment.”
Pav Kalsi, clinical adviser at Diabetes UK, said: “This new report draws attention to a serious issue concerning the misdiagnosis, classification and coding of diabetes in the UK.
“It is absolutely vital that people are diagnosed correctly so they can receive the best course of treatment and care.
“Diabetes is a serious condition and, if untreated or not diagnosed early, it can lead to devastating complications including heart disease, stroke, amputation and blindness.
“However, by highlighting this concern, we can now pinpoint the most effective methods of tackling it to ensure that GPs and clinicians have access to appropriate guidelines and support to reduce the number of people misdiagnosed.”