The nutritional make up of fast food encourages people to gorge on it unintentionally, increasing their risk of obesity, research suggests.
Experts at the Medical Research Council found most fast food is very dense in calories - you only need a small amount to bump up your calorific intake.
They found that these "energy dense" foods can fool people into consuming more calories than the body needs. The research is published in the journal Obesity Reviews.
A typical fast food meal has a very high energy density. It is more than one and a half times higher than an average traditional British meal and two and a half times higher than a traditional African meal.
The researchers concluded that a diet high in fast foods will increase a person's risk of weight gain and obesity - even though they may feel that they are eating no more than they would if they ate an average meal.
Researcher Professor Andrew Prentice, of the London School of Hygiene and Tropical Medicine, said: "We all possess a weak innate ability to recognise foods with a high energy density.
"We tend to assess food intake by the size of the portion, yet a fast food meal contains many more calories than a similar-sized portion of a healthy meal.
"Since the dawn of agriculture, the systems regulating human appetite have evolved for the low energy diet still being consumed in rural areas of the developing world where obesity is almost non-existent.
"Our bodies were never designed to cope with the very energy dense foods consumed in the West and this is contributing to a major rise in obesity."
Professor Prentice drew particular attention to the consequences of a diet high in fast foods for children.
"Children have not yet developed any of the learned dietary restraint that needs to be exerted by anyone wishing to remain slim in the modern environment.
"It's surely a stark paradox that the strategy used to achieve rapid weight gain in malnourished children in Africa - the frequent offering of energy-dense foods - has now become the norm for many overweight children in affluent societies."
Dr Susan Jebb, of the MRC Human Nutrition Research Centre, said: "In many outlets, the choice is so limited that it's virtually impossible to select a combination of items with even a moderate energy density.
"You'd need to eat well below the portion size offered to avoid greatly exceeding recommended energy and fat requirements.
"Fast food companies could play a major part in halting the rise in obesity if they adopted a more positive attitude to healthy eating such as providing meals of lower energy density, appropriately marketed and with point-of-sale nutrition labelling."
Dr Jebb said many supermarket ready-meals and convenience foods were also very energy dense.
"If we're going to stem the tide of obesity, it's important that we don't just swap one unhealthy meal for another.
"Research has shown time and again that to maintain a healthy weight, we need to eat foods with less fat and added sugars and to take more exercise."