More than a third of adults in England have the condition, also known as pre-diabetes, new research suggests.
People classed as having borderline diabetes are at high risk of developing diabetes and its associated complications.
The prevalence of pre-diabetes in England has tripled in the space of eight years, according to the researchers, who described the rise as "disturbing".
People from poorer backgrounds especially were found to be at "substantial risk" of developing diabetes, they added.
Their study, published in the journal BMJ Open, found that in 2011, 35.3% of people had pre-diabetes - up from 11.6% in 2003.
The authors of the study examined data from the Health Survey for England for the years 2003, 2006, 2009 and 2011 involving thousands of participants.
People were classed as having pre-diabetes if their glycated haemoglobin - a measure of blood glucose control - was between 5.7% and 6.4% and they had not previously been diagnosed with diabetes.
The researchers said: "In the absence of concerted and effective efforts to reduce risk, the number of people with diabetes is likely to increase steeply in coming years."
They added: "This rapid rise in such a short period of time is particularly disturbing because it suggests that large changes on a population level can occur in a relatively short period of time.
"If there is no coordinated response to the rise in pre-diabetes, an increase in numbers of people with diabetes will ensue, with consequent increase in health expenditure, morbidity and cardiovascular mortality."
And they warned the findings were particularly problematic given the strong association of pre-diabetes with obesity and being overweight.
Barbara Young, chief executive of Diabetes UK, said: "Having high enough blood glucose levels to be classified as having pre-diabetes leaves people at a significantly increased risk of developing type 2 diabetes."
The lifelong condition that already affects more than three million people can lead to serious health complications such as heart disease, stroke, amputation and blindness.
Ms Young said: "We need to make sure those at high risk are made aware of this so that they can get the advice and support they need to make the lifestyle changes that can help reduce this.
"In fact, up to 80% of cases of type 2 diabetes could be avoided or delayed by making these kinds of changes."
Ms Young said programmes such as the NHS Health Check are already assessing people's risk.
"But at the moment not everyone who is eligible for this check is getting one and we need this to change," she added.
People can also check their risk online or at a high street pharmacy or on the Diabetes UK website: www.diabetes.org.uk/risk.
- Related Stories
- NHS Urged To Send Obese To Slimming Clubs
- Healthier Lifestyles 'Could Stop 37m Deaths'
- Coffee-Drinking 'Can Cut Diabetes Risk'
- Chocolate And Red Wine 'Can Beat Diabetes'
- Sugar In Doctors Sights After Salt Intake Cut
- Tom Hanks Diagnosed With Type 2 Diabetes
- Diabetes Will Create 'Public Health Disaster'