At least one in three children under five are either undernourished or overweight, and one in two lack essential vitamins and nutrients, the UN children’s agency has warned.

The Unicef report laid bare the alarming rate at which poor diets and a “failing” food system are damaging children, saying that “millions are eating too little of what they need and millions are eating too much of what they don’t need: poor diets are now the main risk factor for the global burden of disease”.

In the UK, the situation is a growing crisis. Almost two million children in England live in food poverty and one in three are overweight or obese by the time they leave primary school, Unicef said.

Globally almost 200 million children under five are malnourished, mostly due to poverty and deprivation, while 340 million suffer from hidden hunger in the form of vitamin and mineral deficiencies.

Poor children are carrying “the greatest burden of all forms of malnutrition”, the agency said, stressing that only one in five children aged six months to 23 months from the poorest families “is fed the minimum recommended diverse diet for healthy growth and brain development”.

Facebook Twitter Pinterest The Central Africa Republic is one of the countries with severe levels of child malnutrition. Photograph: Vlad Sokhin/UNICEF

Henrietta Fore, Unicef’s executive director, said the world was losing ground in the fight for healthy diets. “Despite all the technological, cultural and social advances of the last few decades, we have lost sight of this most basic fact: if children eat poorly, they live poorly,” she said.

“Millions of children subsist on an unhealthy diet because they simply do not have a better choice. The way we understand and respond to malnutrition needs to change: it is not just about getting children enough to eat; it is above all about getting them the right food to eat. That is our common challenge today.”

The survival and physical and cognitive development of children is threatened by what Unicef described as “a triple burden of malnutrition” – a combination of undernutrition, hidden hunger from nutrient deficiencies, and obesity among children under five.

“An increasing number of children and young people are surviving, but far too few are thriving because of malnutrition,” the agency said.

In south Asia 50% of children are undernourished or overweight. The malnutrition rate in east and southern Africa is 42%, and 39% in west and central Africa.

Facebook Twitter Pinterest Unicef has warned that iron deficiency reduces children’s ability to learn. Photograph: Vincent Tremeau/Unicef

“Hidden hunger harms children and women,” Unicef said. “Iron deficiency reduces children’s ability to learn and iron deficiency anaemia increases women’s risk of death during or shortly after childbirth.”

Globally, 149 million children under five are stunted, meaning they have low height for their age, and almost 50 million are wasted, with low weight to height ratio.

Being too thin for their height can be lethal for children. Most cases, Unicef said, are in Asia, where wasting affects more than one in seven children under five.

“The number of stunted children has declined in all continents, except in Africa, while the number of overweight children has increased in all continents, including in Africa,” the report said.

Obesity is rapidly rising among children and young people around the world, driving early outbreaks of type 2 diabetes. At least 40 million children over the age of five are overweight. From 2000–2016, the number of overweight children aged five to 19 has doubled from one in 10 to one in five.

“Ten times more girls and 12 times more boys in this age group suffer from obesity today than in 1975,” the agency said.

Poorer children in the UK are twice as likely to be obese compared with those from the wealthier backgrounds. So-called “food swamps” – areas abundant in high-calorie, low-nutrient, processed foods – are disproportionately concentrated in deprived areas. In England, less than one in five children aged five to 15 eat five portions of fruit and vegetables a day and the poorest areas have five times more exposure to fast-food chains and corner shops.

In the UK, Unicef said 1.2 million people living in deprived areas are in what it calls “food deserts”, neighbourhoods without healthy and affordable food options.

The agency expressed concern about the rising use of breastmilk substitutes: globally only two in five children under six months are exclusively breastfed.

The sale of breastmilk substitutes worldwide rose by 41% from 2008–2013, and by 72% in upper-middle-income countries such as Brazil, China and Turkey. Globally, 44% of children aged six to 23 months are not fed fruit or vegetables, and 59% don’t eat eggs, dairy, fish or meat, Unicef said.

Facebook Twitter Pinterest A mass breastfeeding protest at an Extinction Rebellion demonstration in London. Photograph: Peter Summers/Getty Images

The report also looks at how globalisation, urbanisation and the climate emergency are compounding unhealthy diets. Just 100 giant firms dominate 77% of global sales of processed food. “Climate shocks, loss of biodiversity and damage to water, air and soil are worsening the nutritional prospects of millions of children and young people, especially among the poor,” said the report.

In Bangladesh alone, up to 19 million children are on the frontline of climate disasters.