Public Health England said there was a “very high risk” of diabetes with waistlines over 40in (102cm) in men or 35in (88cm) in women.
It warned that the disease could “cripple” the NHS, 10% of whose budget was already spent on it.
The charity Diabetes UK said the country was facing a “devastating” type 2 diabetes epidemic.
Type 2 diabetes is an inability to control blood sugar levels that has dire consequences for health.
It increases the risk of heart attacks and strokes, is the leading cause of blindness in people of working age, damages blood vessels and nerves and results in 100 foot amputations each week in the UK.
If someone has type 2 diabetes at the age of 50, they can expect to die six years earlier than someone without the disease.
Obesity is the biggest risk factor driving the disease.
Public Health England (PHE) says men with a 40in (102cm) waist are five times more likely to get type 2 diabetes than those with a slimmer waistline.
Women were at three times greater risk once they reached 35in (88cm).
The PHE report also warns men with a 37-40in waistline (94-102cm) or women at 31-35in (80-88cm) may not be in the most dangerous group, but still faced a “higher risk” of the disease.
Dr Alison Tedstone, the chief nutritionist at Public Health England, said obesity was now so prevalent “we don’t even recognise it”.
She urged people to “keep an eye on your waist measurement” as losing weight was “the biggest thing you can do” to combat the disease.
Earlier this month, the National Institute of Health and Care Excellence said people with type 2 diabetes should have weight loss surgery.
However, many people mistakenly think their trouser size counts as their waistline, conveniently forgetting about a bothersome beer belly.
Dr Tedstone told the BBC: “People get it wrong, particularly men.
“They measure their waist under their bellies, saying they haven’t got fatter because their trouser size is the same, forgetting they’re wearing their trousers lower and lower.
“So the tip is to measure across the belly button.”
A different form of diabetes – type 1 – is caused by the body’s own immune system rebelling and destroying the cells needed to control blood sugar.
About 3.2 million people have been diagnosed with some form of diabetes in the UK and that figure is projected to reach five million by 2025.
The NHS already spends a 10th of its budget on the diseases.
“That’s a huge amount of money and that could possibly double over the next few years, and that could cripple the NHS,” said Dr Tedstone.
Baroness Barbara Young, the chief executive of Diabetes UK, said: “With many millions of people in the UK now at high risk of type 2 diabetes, this is an epidemic that looks likely to get even worse, and if this happens then the impact on the nation’s health would be devastating and the increase in costs to the NHS would be unsustainable.”
She said the government needed to intervene.
“It needs to urgently consider making healthy food more accessible through taxation, other financial measures and more robust regulation of the food industry,” she said.