Rates of bowel cancer are rising among younger adults, with no clear reason why, experts are warning.

Although the disease still affects older people far more often, the increase in many countries among the under-50s is concerning, Cancer Research UK says.

England is one the countries with the biggest rise, averaging a 3.6% increase per year, researchers report in the journal Lancet Oncology.

Poor diet and obesity may be among the risk factors involved, the researchers say.

The study, of 50 countries between 2007 and 2017, found 27 had seen a rise in cases among younger people.

Many were richer nations but some were developing countries, the investigators, from the American Cancer Society, say.

“The increase in early-onset colorectal cancer is a global phenomenon,” one of the authors, Dr Hyuna Sung, said.

“Previous studies have shown this rise in predominately high-income Western countries – but now, it is documented in various economies and regions worldwide.”

And people needed to be aware of the early signs and symptoms, such as blood in poo.

Other symptoms can include:

• a persistent change in bowel habit – more frequent, with looser stools
• abdominal pain, discomfort or bloating

Cancer Research UK stressed rates in younger adults remained low, with about one in 20 bowel cancers in the UK diagnosed in under-50s.

Of the 44,100 new cases every year in the UK, about 2,600 were in 25-49-year-olds.

Spokesman Jon Shelton said: “The number of early-onset bowel-cancer cases for both men and women each year in the UK is very low, therefore it’s difficult to say for certain why we are seeing rates increasing at a faster rate in one group over another.”

But risk factors such as diet, obesity, alcohol and smoking could be involved.

“Improvements to detection meaning that people are diagnosed at a younger age could also be playing a key role in higher rates,” Mr Shelton added.

Dr David Robert Grimes, an expert in biostatistics, at Trinity College in Dublin, said more research was needed.

“This finding is interesting – but we have to resist the urge to jump to conclusions, especially with conflicting and complicated data,” he said.

“While the headline might seem worrying in isolation, it could also mean that screening and improved detection are allowing us to catch cancers earlier.”

Dame Deborah James, who died from bowel cancer, aged 40, had received widespread praise for sharing her experiences on social media and urging people to “check their poo” and get tested if they noticed any irregularities in their bowel movements.

The mother of two, known to many as Bowelbabe, also hosted BBC Sounds’ You, Me and the Big C podcast and made a documentary about living with the disease.