The campaign group Action on Sugar is demanding a ban on freakshakes and all milkshakes with more than 300 calories.
It surveyed milkshakes sold in restaurants and fast food shops in the UK and found they contained “grotesque levels of sugar and calories”.
Freakshakes are milkshakes that also contain chocolates, sweets, cake, cream and sauce.
The Toby Carvery Unicorn Freakshake came top of the survey with 39 teaspoons of sugar or 1,280 calories.
That is more than half the daily recommended amount of calories for an adult and over six times the amount of sugar recommended for seven to 10-year-olds.
In terms of sugar, it is the equivalent of drinking more than five cans of cola.
Action on Sugar chairman Graham MacGregor, professor of Cardiovascular Medicine at London’s Queen Mary University, said that does not go far enough.
“These very high calorie drinks, if consumed on a daily basis, would result in children becoming obese and suffering from tooth decay – that is not acceptable,” he said.
“These high calorie milkshakes need to be reduced immediately below 300kcal per serving.”
The Toby Carvery Unicorn Freakshake is made with grape and raspberry flavoured ice cream, blackcurrant jelly pieces, milk, freshly whipped cream and topped with marshmallow, skittles and almond macaroon.
A Toby Carvery spokesperson added: “We also regularly work with our suppliers to explore ways we can reduce sugar levels in our dishes.”
And a Harvester spokesperson said: “We have also been recognised by the Soil Association Out to Lunch campaign for continuing to develop our children’s menu and offering healthier choices for young guests.”
Action on Sugar also looked at supermarket milkshakes and found that 90% of the 41 products surveyed would receive a red/high label for excessive levels of sugars.
The top three for sugars and calories were Muller Milk Frijj in chocolate, fudge brownie and cookie dough flavour.
A spokesperson for Muller said: “We know that for health conscious consumers, the choice of reduced sugar is important, without compromising on taste. That’s why we introduced a Frijj zero added sugar range.
“Milk contains naturally occurring sugars in the form of lactose but we would stress that it is also a natural and nutrient rich product, containing essential nutrients such as protein, calcium and vitamins.”