It will come as no surprise that milkshakes loaded with sweets, chocolate bars, cream and cake, known as freakshakes, contain "grotesque levels" of sugar.
But now one campaign group wants them to be labeled with a nutrition warning and the worst offenders banned.
Business and industry sectors
Business, economy and trade
Food and beverage industry
Food and drink
Kinds of foods and beverages
Restaurant and food service industry
Continents and regions
Diet and nutrition
Diet, nutrition and fitness
Families and children
Family members and relatives
Health and health care (by demographic group)
Health and medical
Maternal and child health
Population and demographics
Action on Sugar, based at Queen Mary University of London in the UK, conducted a survey of 140 drinks at UK fast-food chains and restaurants and found they contained "alarming" levels of sugar and calories.
The "Unicorn Freakshake" sold by UK restaurant chain Toby Carvery was the worst of those surveyed, containing more than six times the recommended daily amount of sugar for children between the ages of 7 and 10.
At 1,280 kilocalories it contains more than half the recommended 2500 calorie intake for an adult.
"An average 25-year-old would need to jog for nearly three hours or vacuum the house for five hours to burn off the calories!" the group said in a statement.
The drinks are popular on Instagram, where the general rule is that the bigger the tower of sugary items, the more delicious the drink.
The next worst offender on Action on Sugar's list was Five Guys' banana and chocolate shake with 1,073 calories -- the equivalent of more than four cans of cola. The fast-food chain also took third position for its cherry milkshake, which has 975 calories.
Researchers considered products from 14 chains of restaurants and fast-food outlets. Others that were surveyed included Handmade Burger Company and TGI Fridays, both of which were among several companies that do not publish nutrition information about their shakes.
The UK has been battling the rising tide of childhood obesity, with almost a third of children aged 12-15 overweight or obese. This is not only associated with social and psychological problems but also a host of serious diseases such as diabetes, cardiovascular disease and certain types of cancer.
In August 2016, Public Health England (PHE) set out its approach to reduce the prevalence of childhood obesity by launching a structured sugar reduction program to remove sugar from everyday products. All sectors of the food and drinks industry were challenged to cut sugar content by at least 20% by 2020.
"Despite milkshakes being incorporated into Public Health England's sugar reduction program as part of the government's childhood obesity plan, it is clear from our survey that much more needs to be done than a 20% reduction, said Graham MacGregor, professor of cardiovascular medicine at Queen Mary University of London and chairman of Action on Sugar. "These very high-calorie drinks, if consumed on a daily basis, would result in children becoming obese and suffer from tooth decay -- that is not acceptable."
The group is urging health authorities to go further by calling for mandatory traffic light colored nutrition labeling across all menus. In addition, the group has called for an outright ban on the sale of milkshakes containing more than 300 calories per serving.
According to the charity's research, nine out of 10 milkshakes sold in UK supermarkets would be eligible for a red label for the excessive level of sugar they contain. Nevertheless, these products contained far less sugar than the freakshakes sold in fast-food outlets.
"Milkshakes can contain significant amounts of sugar," said Alison Tedstone, chief nutritionist at PHE in response to the report. "The food and drink industry -- including restaurants, manufacturers and retailers -- has a key role in helping to tackle this by reducing the amount of sugar we buy and consume, and we hope to see them step up to the challenge."
A spokesman for Toby Carvery said: "Freakshakes only feature on our main menu and are not targeted at children."
The chain is "very mindful of our role in helping guests make informed decisions" and makes public the nutrition information for its menu, the spokesperson said. The company has also committed to PHE's sugar reduction program.
Spokespersons for Five Guys weren't immediately available for comment.