Up to five million people in England are at risk of developing type-2 diabetes, according to new data from Public Health England.
Type-2 diabetes is closely linked to diet and obesity and affects about 3.2 million people across the UK.
The NHS says diabetes causes 22,000 early deaths and costs the health service more than £8bn each year.
But health experts believe more than a quarter of people can reduce the risk of developing the condition.
Diabetes arises when the body loses the ability to use or make insulin, a hormone that helps regulate the amount of sugar in blood.
Public Health England (PHE) says its latest analysis shows about five million adults in England are now pre-diabetic, also known as non-diabetic hyperglycaemia.