Type 2 is the most common form of diabetes
An estimated seven million people in the UK have early warning signs of diabetes, a charity has warned.
People with pre-diabetes have higher than normal blood sugar levels and a 12 times increased risk of developing full-blown type 2 diabetes.
Diabetes UK called pre-diabetes, which can cause long-term damage but is reversible with exercise, weight loss and healthy diet, a "ticking timebomb".
A total of 2.6 million people in the UK have diabetes, the latest figures show.
Also known as impaired glucose regulation, pre-diabetes tends to occur in people who are overweight or obese and most people have a family history of diabetes or also have high blood pressure or high cholesterol.
Diabetes UK said recent research had shown pre-diabetes could cause long-term damage to the body, especially the heart and circulation, even before full-blown diabetes develops.
But although doctors view the condition as an early warning sign of diabetes it is not a foregone conclusion and the risk can be cut by 60% by losing weight, eating a healthy balanced diet and doing more exercise, the charity said.
Recent figures show that more than 145,000 new cases of mainly type 2 diabetes were diagnosed in the past year.
In April this year, the government launched a programme of health checks for people aged between 40 and 79 who are not already diagnosed with heart disease, diabetes, kidney disease or have had a stroke.
The screening is designed to pick up risk factors for the diseases such as being overweight and high blood pressure.
Diabetes UK chief executive, Douglas Smallwood, said pre-diabetes was a "ticking timebomb".
"It's staggering that seven million people in the UK have pre-diabetes, which is often a precursor to type 2 diabetes, a serious condition which can lead to long-term complications such as heart disease, stroke, kidney failure, amputation and blindness.
THOSE AT RISK
Anyone who is white and over 40 or black or South Asian and over 25 with the following risk factors:
Close relative with type 2 diabetes
Overweight or waist size over 31.5 inches for women; 37 inches for men, or 35 inches or over for South Asian men
High blood pressure or previous heart attack or stroke
Polycystic ovary syndrome or previous gestational diabetes in women
Severe mental health problems
"Identifying and educating people with pre-diabetes is vital as it's not too late for many to make healthy lifestyle changes, reverse the condition completely and reduce their risk of developing type 2 diabetes."
Dr Brian Karet, a Bradford GP and diabetes spokesman for the Royal College of GPs, said the idea of pre-diabetes was not a new one but one with which the general public might not be familiar.
"People in general need to realise that small increases in their weight lead to big problems in the future.
"Small improvements in diet and small increases in exercise will pay dividends."
A spokesman for the Department of Health said it was on track to deliver one million health checks this year.