Dr Rachel Huxley from the University of Sydney and her team looked at 18 studies involving nearly half a million coffee drinkers. Six studies also monitored decaffeinated coffee and seven reported on tea drinking. They report their findings in the Archives of Internal Medicine.

When the researchers combined the results and analysed the data, they found that each additional cup of coffee drunk in a day was linked to a 7% reduction in the risk of diabetes. Drinking three or four cups a day had around a 25% lower risk than those who drank between two cups per day and no tea or coffee.

The researchers found that with decaf coffee, people who drank more than three to four cups a day had about a one-third lower risk of diabetes than those who drank none.  

Because both normal coffee and decaf seemed to help, and decaf gave better results, the researchers say the association is unlikely to be just related to caffeine. Other compounds in coffee and tea including magnesium, antioxidants known as lignans or chlorogenic acids may be involved, they say.

Although the Australian study looked at tea and coffee linked to the risk of diabetes, earlier studies suggested that caffeinated drinks can be bad for people who already have diabetes.

In 2008, a US study found caffeine makes it harder for people with diabetes to control their blood sugar. The Duke University research found that when patients took caffeine, their blood-sugar levels were 8% higher. And after every meal -- including dinner -- their blood sugar spiked higher than it did on days when they had no caffeine.

More than 4% of the UK population has a form of diabetes. The NHS says diabetes affects two million people in England and Wales and it is thought that there are another 750,000 people who have the condition but are unaware of it.

Identifying these active components could lead to new treatments being developed. But will people at risk from diabetes be asked to add a few extra lattes or cappuccinos to their diet and exercise plans?

Dr Victoria King, Research Manager at the charity Diabetes UK, told us in a statement by email: "Without full information about what other factors may be influencing the type 2 diabetes risk of the studies' participants - such as their physical activity levels and diet - as well as what the active ingredient in tea or coffee appears to be, we cannot be sure what, if anything, this observed effect is down to.

"What we can be sure of is that the development of type 2 diabetes is strongly linked to lifestyle, which means that many cases could be prevented by keeping active and eating a healthy balanced diet that is low in fat, salt and sugar with plenty of fruit and vegetables."