Coffee chains Starbucks and Costa promised to come up with healthier drinks on Wednesday after a British campaign group found "dangerously" high sugar levels in some of their hot flavoured beverages.

Out of 131 drinks analyzed by Action on Sugar, 98 per cent were found to have high enough sugar content to merit a red nutritional label, which in Britain indicates food that consumers "should cut down on".

U.S. chain Starbucks' extra large Grape with Chai, Orange and Cinnamon Hot Mulled Fruit contains 99 grams (3.5 ounces) of sugar per serving -- the equivalent of 25 teaspoons, the charity said.

British firm Costa's large Chai Latte was found to contain 20 teaspoons of sugar, while both KFC's Mocha and the extra large Starbucks Signature Hot Chocolate had 15 teaspoons.

"This is yet again another example of scandalous amount of sugar added to our food and drink," said Graham MacGregor, chairman of Action on Sugar.

"No wonder we have the highest rates of obesity in Europe."

A Starbucks spokeswoman said the company had committed this year "to reduce added sugar in our indulgent drinks by 25 per cent by the end of 2020".

"We also offer a wide variety of lighter options," she said.

Costa's head of communications Kerry Parkin said the company had taken "significant steps to reduce the sugar content of our ranges" and would "continue improving the balance of our product offerings".

But Kawther Hashem, a researcher with Action on Sugar, said coffee shop chains should act "immediately" to reduce the amount of sugar used, improve labelling and stop selling extra-large sizes.