More than half the world's population will be classed as obese or overweight by 2035 if action is not taken, the World Obesity Federation warns.

More than four billion people will be affected, with rates rising fastest among children, its report says.

Low or middle-income countries in Africa and Asia are expected to see the greatest rises.

The report predicts the cost of obesity will amount to more than $4tn (£3.3tn) annually by 2035.

The president of the federation, Prof Louise Baur, described the report's findings as a clear warning to countries to act now or risk repercussions in the future.

The report in particular highlights the rising rates of obesity among children and teenagers, with rates expected to double from 2020 levels among both boys and girls.

Prof Baur said the trend was "particularly worrying", adding that "governments and policymakers around the world need to do all they can to avoid passing health, social, and economic costs on to the younger generation" by assessing "the systems and root factors" that contribute to obesity.

The effects of obesity's prevalence on lower-income countries is also highlighted in the report. Nine of the 10 countries with the greatest expected increases in obesity globally are low or lower-middle income states in Africa and Asia.

Reasons include trends in dietary preferences towards more highly processed foods, greater levels of sedentary behaviour, weaker policies to control food supply and marketing, and less well-resourced healthcare services to assist in weight management and health education.

Lower-income countries are "often the least able to respond to obesity and its consequences".

The findings estimate that rises in obesity rates around the world will have a significant impact on the global economy, equating to 3% of global Gross Domestic Product.

The report emphasises that its acknowledgement of the economic impact of obesity "is in no way a reflection of blame on people living with obesity".

The data published in the report will be presented to the UN on Monday.

Obese is a medical term used to describe a person with a high excess of body fat.

The report uses body mass index (BMI) to make its assessments. BMI is calculated by dividing an adult's weight by the square of their height.