They've been hailed a dieter's worst enemy and become the most maligned vegetable around.

Now, doctors have warned potatoes may also raise the risk of type 2 diabetes.

Eating seven or more servings of spuds a week increases the risk of the disease by 33 per cent, compared to less than one full serving, a study found.

And even two to four portions increases the risk by 7 per cent, researchers say.  

Unsurprisingly, French fries are the worst culprit compared with baked, boiled or mashed potatoes.

However replacing three servings of potatoes a week with wholegrains - such as rice, quinoa, wheat and corn - could lower the risk of type 2 diabetes by 12 per cent, researchers concluded.

The study's author, Dr Isao Muraki, of the Osaka Center for Cancer and Cardiovascular Disease Prevention, told HealthDay that although potatoes are considered a vegetable in some countries, they should 'not be regarded as a key component of a healthful diet'.

Rather, they should be thought of as similar to a refined carbohydrate.

Dr Muraki, who also works for the Harvard TH Chan School of Public Health, continued: 'Potatoes contain a large amount of starch and a relatively small amount of fibre, vitamins, minerals, and polyphenols [disease-fighting chemical compounds found in fruits and vegetables].

'Lower quality and quantity of carbohydrate is associated with a higher risk of type 2 diabetes.'

And when potatoes are served hot, their starch becomes more easily digestible and raises blood glucose levels more quickly, he explained.

In the US national food guide, by the Institute of Medicine, potatoes are considered a 'healthful vegetable'.

But with the UK equivalent, the Eatwell plate, potatoes are listed as a cereal.

To come to their conclusions, Dr Muraki and his team combined data from three US studies on 70, 773 nurses and 40,669 male health professionals.

Those involved did not have diabetes, cardiovascular disease (including heart disease and or history of a stroke), or cancer at the start of the study.

The team assessed how many servings of potato people ate a week using questionnaires - excluding crisps, or potato chips.

After four years, the researchers looked at data for potato consumption and type 2 diabetes.

They found people who ate more - either in the form of baked, boiled, mashed or French fries - had a 'significantly' higher risk of type 2 diabetes.

This was still the case after taking into account factors such as lifestyle, diet and body mass index.

Adjusting for glycemic load - a measure of how much the food will raise a person's blood sugar - did not change the results either.

Overall, people who increased their consumption of potatoes over time — especially French fries — had an increased risk of type 2 diabetes.  

'Potato food consumption should be reduced for people with elevated risk of developing diabetes or insulin resistance,' Dr Muraki said.

'Healthy foods such as vegetables, fruits, whole grains, and nuts - rather than potatoes - should be encouraged for the prevention of type 2 diabetes.'

However, he noted that because study looked at data for health professionals of European ancestry, the results may not extend to other ethnic and demographic groups.

The research was published online in the journal Diabetes Care.