The average Briton consumes 50 per cent more calories than they realise, according to the first estimates from the Office for National Statistics.
The new data show that men are the worst at kidding themselves - typically consuming 1,000 more calories than they estimate every day - while women eat about 800 calories more than they account for.
And the more people eat, the less reliable their estimates are, the research found.
Experts said the delusion occurs because people do not like to “be taken for slobs” - and ended up lying to themselves.
The revelation comes as Public Health England (PHE) prepares to launch a calorie counting campaign, as part of efforts to combat Britain’s obesity epidemic.
Two thirds of adults are overweight or obese, along with one third of children by the time they leave primary school.
New data suggest that the crisis is fuelled by the fact the average person wildly underestimates the amount of calories they are consuming daily.
The survey of more than 4,000 adults asked men and women to attempt to track their calorie consumption over four days.
The research - which tracked how many calories participants expended, as a proxy of their likely intake, found a massive gulf between estimated consumption and the number of calories expended.
On average, the figures from men amounted to an average daily consumption of 2,065 calories, while the average for women was 1,570.
Men were found to expend an average of 3,119 calories - meaning that was their likely consumption level - while women were expending 2,393 daily.
The more overweight consumers were, the less reliable their account of their calorie intake was, the research found, while women were slightly more accurate than men.
Tam Fry, from the National Obesity Forum, said: “People lie and I am not surprised that they do when it comes to food. They wish not to be taken for slobs, even though they may be just that.”
And he said new guidance on calorie counting, due out in March, was an “absolutely ridiculous” attempt to solve the problem as it was unrealistic.
The new PHE advice, in the One You nutrition campaign, will say adults should limit lunches and dinners to 600 calories each, with 400 calories for breakfast.
Those behind the campaign say overall recommended daily consumption levels are unchanged- at 2000 calories for women and 2500 for men - but that the guidance is a “rule of thumb” to help people cut back.
Health officials say the average adult is consuming 200 to 300 more calories every day than they should.
But the new figures suggest this may be a significant underestimate, explaining the growing obesity crisis.
Britain is the fattest country in Western Europe, international research shows, with rates of obesity rising even faster than those in the United States.
Research has attributed rising obesity levels to changes in diet, with increasing portion sizes, far more meals eaten outside the home and a shift to ready meals, junk foods and snacks.
In the 1970s, less than 3 per cent of adults in England were obese, compared with 25 per cent now.
Experts say that although activity levels have fallen slightly over the period, most of the weight gain is blamed on the rising amount of high-calorie foods consumed.
Habits which were once occasional treats - like takeaways and restaurants - have become routine, they say. Today British families eat out at least twice as much as they did in the 1970s, with at least one in five meals now eaten outside the home.
Analysis by the Organisation for Economic Co-operation Development published last year shows levels in the UK have risen by 92 per cent in just over two decades - by far the steepest rise among countries with an obesity problem.
The Office for National Statistics said the new findings are the results of experimental research, and should not be interpreted as official statistics, with further testing and validation required.
Professor John Wass, a consultant endocrinologist and spokesman for the Obesity Health Alliance, said: “The technology used in this study to measure calorie intake is watertight, and the findings confirm what we already know. One of the main contributing factors in the obesity epidemic facing this country is people consuming too many calories.
“The fact that people appear to be underestimating their calorie intake is not surprising. This isn’t necessarily about being in denial, but demonstrates the difficulty in calculating the nutritional content of food.”
He said the efforts of dieters to lose weight, and of others to maintain a healthy weight were being compromised by a lack of clear labelling, especially when people were eating out.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: “Underreporting of intakes has always been a feature of all diet surveys. Some people forget what they’ve consumed and some change what they record knowing they are part of a survey.
“There’s no way to get rid of underreporting, but the steps we take to minimise it makes the National Diet and Nutrition Survey the most robust data on the population’s diet.”
Source The Telegraph, February 2018