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$\textbf{Background}$: Canine diabetes mellitus (DM) is a common endocrinopathy with a complex genetic architecture. Disease susceptibility in several breeds has been found to be associated with polymorphisms in immune response genes, but in the Labrador retriever breed no genetic associations with DM have been identified to date. Recently, a deletion in the pro-opiomelanocortin ($\textit{POMC}$) gene has been reported in Labrador retrievers, which has been associated with increased appetite and risk of obesity in this breed. $\textbf{Hypothesis / Objectives}$: To characterise the POMC deletion in Labrador retrievers, to develop a simple genetic test for this mutation and to test the hypothesis that the $\textit{POMC}$ gene deletion is associated with an increased risk of DM in this breed. $\textbf{Animals}$: Sixty one non-diabetic Labrador retrievers aged >6 years and fifty seven Labrador retrievers affected with DM. $\textbf{Methods}$: Case-control genotyping study to compare the frequency of the $\textit{POMC}$ deletion in dogs with and without DM. Following polymerase chain reaction and sequencing to characterise the mutation, a PCR-based test was developed and validated using two different restriction fragment length polymorphism assays. $\textbf{Results}$: A 14-base-pair deletion was confirmed and localised to exon 3 of the canine $\textit{POMC}$ gene. A PCR-based test for the deletion was successfully developed. There was no association between the presence of the $\textit{POMC}$ deletion mutation and DM in this population of Labrador retriever dogs. $\textbf{Conclusions and clinical importance}$: This study adds to the existing scientific literature indicating that there is little evidence for a direct link between obesity and DM in dogs.