Nuffield Health, the UK's largest healthcare charity, analysed Health MOT data on more than 54,000 people who were trying to improve their health and fitness.
They found that the average women had a waist that measured 4.9cm more than the healthy maximum of 80cm, while the average man had a waist size in the healthy range.
More than half of the women (52.5 per cent) also had a body mass index (BMI) that exceeded the healthy range of 18-25, with 16.2 per cent registering in the obese range.
And the average waist-to-hip ratio was 0.83, which suggests women tend to be more of an 'apple' shape than a 'pear' shape.
This is concerning, as an apple shape is generally thought to be associated with greater health risks.
In fact, Nuffield Health warned that many women could be increasing their chances of developing illnesses and health complications - such as breast, womb and ovarian cancer, heart disease, type-2 diabetes and infertility - by failing to keep an eye on their waistline.
Dr Davina Deniszczyc, professional head of physicians and diagnostics at Nuffield Health, said: "Our Health MOT results provide a fascinating snapshot of some of the health issues facing us as a nation.
"Whilst waist size may seem like a cosmetic issue, this isn't about women fitting into their skinny jeans, rather it's an important indicator of overall health and wellbeing, particularly when taken into account with other health measurements."
Dr Deniszczyc warned that women who fail to keep an eye on their waist could be at risk of ill-health, as excess fat stored around the waist can increase the likelihood of "significant health issues".
If you're concerned about your waistline, why not consider making some small changes to your diet and lifestyle?
You don't have to aim for the perfect bikini body, but try to keep your waist measurement under 80cm and check it every few weeks to make sure you're going in the right direction.
According to Chris Jones, head of physiology at Nuffield Health, you might want to avoid eating certain foods too often, such as white rice, white bread, white pasta and sugar, as these can increase the amount of fat stored around your stomach.
Instead, choose options such as brown basmati rice, whole wheat pasta and lean proteins, and switch to decaffeinated beverages if you're used to drinking more than three cups of tea or coffee per day.