Adults who are obese in Britain for five years will be superior to those who have a healthy weight.
Experts warn that in 2027 there will be a “turning point” when a third of adults will suffer from obesity, if current trends continue.
They predict that by 2040 there will be 21 million obese people in the UK and 19 million overweight.
Analysis by Cancer A UK study shows that seven out of 10 (71 per cent) people will be overweight or obese by 2040. Of these, nearly four out of 10 (36 percent) adults will be obese.
Currently, 64 percent of adults are overweight or obese, and the numbers are growing every year.
The report estimates that the number of obese people may overtake the number of people with a healthy weight as early as the late 2020s in England and the late 2030s in Northern Ireland.
It is expected that for Scotland and Wales the crossover will take place no earlier than 2040.
New figures emerge after former Conservative leader William Hague attacked the government for postponing a ban on banning high-fat, salt and sugar foods for a year due to the cost of living crisis.
The ban on television advertising of junk food before the 21:00 watershed has also been postponed for a year.
Write to The TimesLord Hague noted the weakening of the anti-obesity strategy under pressure from deputies is “morally reprehensible.”
Being overweight or obese increases the risk of at least 13 different types of cancer, and also causes other diseases such as high blood pressure and type 2 diabetes.
Approximately 22,800 cases of cancer in the UK each year are due to overweight and obesity.
Michelle Mitchell, Executive Director of Cancer Research UK, said: “These forecasts should serve as an alarming call for the government about the health of our country.
“Ministers should not continue to rush into the fight against the obesity crisis – postponing measures that will lead to a healthier diet.
“I urge them to reconsider this decision and take bold action to combat obesity, the second largest preventable cancer risk factor in the UK.”
The prevalence of severe obesity (body mass index above 40) in the UK is expected to almost double from 3 to 6 per cent.
According to the report, overweight seems to have become a plateau at the moment, but obesity is still rising.
Among women, about 60 percent are currently overweight or obese, but by 2040 this figure is projected to rise to 67 percent.
Among men by 2040, 74% will be overweight or obese, compared to 68% now.
The report also suggests that those experiencing higher levels of deprivation may suffer the most.
In England, in 2019, 35 per cent of people living in the poorest areas were obese, and it is estimated that by 2040 this figure will rise to almost half (46 per cent).
By comparison, in 2019, 22 percent of people living in the poorest areas were obese, and it is estimated that this figure will rise to 25 percent.
Dr Julie Sharpe, head of Cancer Research UK’s Health and Patient Information Department, said: “Obesity is a complex problem and the world around us can find it very difficult to maintain a healthy weight.
“Government action is key to ensuring that the healthy eating option is easily accessible and accessible to people, and to removing broader barriers that prevent people from leading a healthy lifestyle.
“If these staggering trends continue, obesity will overshadow smoking as the biggest cause of cancer.”
A spokesman for the Department of Health and Human Services said: “We recognize the urgent need to combat obesity, given that it costs the NHS around £ 6.5 billion a year and is the second largest cause of cancer.
“We are taking urgent action to encourage people to choose healthier foods through the introduction of calorie labels in the menu, support for families through schemes such as healthy starting and the introduction of rules for placing less useful products in stores.
“We will identify further actions to improve the health of the nation in the White Paper later this year.”
Additional Press Association reports