Scientists estimated the increase in rates for those aged under 50 between 1990 and 2019.

And the team from the University of Edinburgh and the Zhejiang University School of Medicine in China identified factors such as obesity and alcohol consumption as contributing to the global rise.

However, researchers pointed out that rates in the UK stabilised from 2010 to 2019 - with the annual mortality rate from early-onset cancer "steadily decreasing".

The team behind the study analysed data from the Global Burden of Disease 2019 Study for 29 cancers in 204 countries and regions.

Looking at new cases, deaths, health consequences and risk factors in people aged 14 to 49, they estimated an annual percentage for each year.

There were 1.8 million new cancer diagnoses for under-50s in 2019 - an increase of 79.1% since 1990 - with deaths also up by 27.7%.

Researchers said that while genetics were likely to have contributed to the rise, smoking, alcohol consumption and diets high in meat and salt but low in fruit and milk were the "main risk factors".

Excess weight, low physical activity and high blood sugar were also found to have played a part.

Breast cancer made up the largest proportion of cases - 13.7 per every 100,000 people. Windpipe and prostate cancer cases, meanwhile, are growing the fastest at 2.28% and 2.23% per year respectively.

However, early-onset liver cancer cases fell by 2.88% each year.

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North America, Australasia and Western Europe were the regions with the highest rates of early-onset cancers.

Study author Dr Xue Li, of the Centre for Global Health at the University of Edinburgh's Usher Institute, said early-onset cancer in the UK showed an "upward trend" from 1990 to 2010, but that "the overall incidence rate remained stable" from 2010 to 2019.

"Fortunately, the annual mortality rate from early-onset cancer in the UK has been steadily decreasing, a testament to the outstanding cancer screening and treatment efforts over the past three decades," she added.

The study's publication in the journal BMJ Oncology - comes after charity Cancer Research UK claimed advances in cancer care had helped save 1.2 million lives in the UK since the mid-1980s.

Cause of rise 'not fully clear'

Dr Claire Knight, senior health information manager at Cancer Research UK, said: "It's not fully clear what is driving the rise in early-onset cancers, but exposure to risk factors in earlier life, better detection of cancer and genetics might all play a part."

Dr Knight said cancer remains "primarily a disease of older age", however "alarming" the findings of the study might seem.

"We need more research to examine the causes of early-onset cancer for specific cancer types, like our BCAN-RAY study that is looking at new ways to identify younger women at higher risk of breast cancer," she added.

"If people are concerned about their cancer risk, there are lots of ways to help reduce this, such as not smoking, maintaining a balanced diet, getting plenty of exercise and staying safe in the sun."