Around 100,000 people in England may have been incorrectly diagnosed with diabetes, an audit of GP patient records suggests.
Reviews at five GP practices in Surrey, covering 45,000 patients, found 2% of those diagnosed with diabetes did not have it and another 2% were labelled with the wrong type of diabetes.
Another 1% had been incorrectly logged on surgery computer systems.
Doctors' leaders insist the errors are unlikely to have damaged patient care.
The Royal College of General Practitioners and NHS Diabetes, who have published the evidence in a report, say the overwhelming majority of people are correctly diagnosed with what is increasingly recognised as "a very complex condition".
They have published new guidance aimed at improving the way the disease is recorded.
The findings reveal around 80 out of 1,600 so-called diabetic patients were misdiagnosed.
If the same is true in other regions then it is likely that around 50,000 people in England are diagnosed with diabetes but do not have it, and another 50,000 are classified with Type 2 diabetes when in fact they have Type 1, and vice versa.
And for every 500 people with diabetes on a GP register, about 65 to 70 will need to be looked at again for some sort of error.
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."