As temperatures in Central London reached 35.7C (96F) a record, nutritionists advised slimmers to come off Atkins and other high-protein diets or risk collapsing.

In June, two American medical studies ruled that the Atkins diet, which bans bread, fruit and potatoes in favour of meat, fish and cheese, was nutritionally safe, prompting a surge in the number of followers.

But yesterday a leading member of the British Nutrition Foundation told The Times that they were concerned that the hot weather could exacerbate the negative effects of the diet.

High-protein foods are harder to digest than carbohydrates, meaning that the body has to expend energy which otherwise would be used for keeping cool.

Dr Sarah Schenker, a dietician and spokesman for the foundation, said: “We have respiratory quotas on food, so when a carbohydrate or protein or fat is being metabolised it generates its own heat.

“The body has to work harder to metabolise protein than other food types. This means that on a really hot day, people on the protein-based diet who are facing, say, the Underground could have problems. My advice would be avoid the Atkins diet in hot weather.”

Nutritionists estimate that around 10 per cent of the body’s energy is needed to break down food.

The Atkins diet, devised by Dr Robert Atkins more than 30 years ago, has inspired millions of women worldwide to rise up against the temptation of carbohydrates in favour of a juicy steak with a side order of spinach or rocket.

It recently came back into fashion after a number of celebrity endorsements, including Brad Pitt, Jennifer Aniston, Renee Zellweger and Robbie Williams.

Dr Atkins's book, New Diet Revolution is at No 2 on the bestseller list and is outselling everything but Harry Potter by three to one. Nearly 15 million dieters already own copies of his earlier volumes. The diet warning came as Britain recorded its hottest day for 13 years with Gravesend in Kent recording the day’s high of 36.4C (97.5F). Predictions that the 100F (37.8C) barrier would be broken proved wide of the mark — much to the relief of bookmakers who had taken huge numbers of bets that that record would be broken.

Beach resorts across the country reported bumper takings and scores of people were taken to hospital suffering heat exhaustion. Police warned people not to cool off in lakes and rivers as two teenagers died and dozens were injured in incidents across the country.

At the Met Office headquarters in Bracknell, Berkshire, a spokesman said: “It is 34C here but 35.3C in the middle of London has got to be pretty exhausting. I would not want to be there.”

The spokesman warned that while people would continue to enjoy sunny days, they would have to endure sultry — and sleepless — nights as temperatures refused to fall below 20C after dark.

Temperatures are forecast to drop slightly today as a plume of the warmest air slips across the Channel into the Low Countries.

The spokesman said: “Instead of being very hot it will be very warm. Mid-to-late 20s will be the norm in the South East for the weekend followed by a bit of a blip on Tuesday, when some places could have a shower. But overall the high pressure is looking very settled.”

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