The new classification guidelines have been developed by a working group supported by NHS Diabetes and the RCGP.
Research on patient records, used to underpin the guidelines, showed that around 40% of people classified as having type-1 diabetes actually have type-2 diabetes.
In addition, around 6% of those diagnosed with type-2 diabetes probably do not have the condition, the investigation suggested.
For an average-sized practice this equates to around six misclassified patients and 12 patients misdiagnosed.
The working group is developing an audit tool to help practices identify patients who have been misclassified, so they can be reassessed at their annual reviews.
A draft version of the criteria was unveiled at the Diabetes UK Annual Professional Conference in Liverpool last week.
As well as types 1 and 2, the guidelines include categories for genetic forms of diabetes and for patients whose classification is as yet uncertain.
To be classed as type-1, a patient must have been put on insulin immediately after diagnosis (if aged over 35 years) or within six months of diagnosis (if under 35).
The guidelines include gestational diabetes as a form of non-diabetic hyperglycaemia, alongside impaired glucose tolerance and impaired fasting glucose.