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.
Health officials will next month launch a national campaign, urging Britons to cut back their calories.
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 while 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 the norm, they say. Today UK 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.