Unprecedented numbers of young people are being diagnosed with bowel cancer - due to poor diets and lack of exercise, a study warns.

Millennials - those born between 1980 and 1995 - are four times more likely to develop rectal tumors stemming from the large intestine compared to those born around 1950.

An alarming three in ten rectal cancer diagnoses are now in patients below the age of 55. Young people are also at twice the risk of developing colon tumors, which start growing lower down.

And yet, these diseases are still widely thought of as something that hits after middle age.

The study warns the data should be a warning sign that this generation faces an epidemic of digestive diseases - and suggests we begin screening people in their early 20s, rather than in their 60s.

Crisis? Millennials - those born between 1980 and 1995 - are four times more likely to develop rectal tumors stemming from the large intestine compared to those born around 1950

BOWEL CANCER RISK FACTORS It's not known exactly what causes bowel cancer, but there are a number of things that can increase the risk. These include: Age – almost nine in 10 cases of bowel cancer occur in people aged 60 or over Diet – a diet high in red or processed meats and low in fibre can increase your risk Weight – bowel cancer is more common in people who are overweight or obese Exercise – being inactive increases the risk of getting bowel cancer Alcohol and smoking – a high alcohol intake and smoking may increase your chances of getting bowel cancer Family history – having a close relative who developed bowel cancer below 50 years of age puts you at a greater lifetime risk of developing the condition SYMPTOMS: The three main symptoms are:

Blood in the stools

Changes in bowel habit such as to more frequent

Looser stools

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The outlook is so bad, the experts say, that the situation compares with the late 1800s.

Epidemiologist Dr Rebecca Siegel, of the American Cancer Society, said: 'Trends in young people are a bellwether for the future disease burden.

'Our finding that colorectal (bowel) cancer risk for millennials has escalated back to the level of those born in the late 1800s is very sobering.

'Educational campaigns are needed to alert clinicians and the general public about this increase to help reduce delays in diagnosis, which are so prevalent in young people, but also to encourage healthier eating and more active lifestyles to try to reverse this trend.'

In 2013, 10,400 new cases of colorectal cancer were diagnosed in people in their 40s, with an additional 12,800 cases diagnosed in people in their early 50s.

Added Dr Siegel: 'These numbers are similar to the total number of cervical cancers diagnosed, for which we recommend screening for the 95 million women aged 21 to 65 years.'

In the UK, screening for bowel cancer is available to those over the age of 60.

Until now, bowel cancer - called colorectal cancer because it starts in the colon or the rectum - has been mainly a disease of the elderly. In Britain, almost nine out of 10 people with the disease are over 60 years old.

Previous research has found snacking on fast food, chocolate, cakes and soda could increase the risk of the disease.

A diet high in red or processed meats, like bacon and sausages, and low in fiber increases the risk, as is being overweight or obese or being inactive. A high alcohol intake and smoking has also been linked with the disease.

The study published in the Journal of the National Cancer Institute said the number of cases is rising in young and middle aged adults, including people in their early 50s, with rectal cancer rates increasing particularly fast.

Overall, bowel cancer has been declining in the US since the mid 1980s, with steeper drops in the most recent decade driven by screening.

However, young diagnoses are increasing, and more young people are engaging in lifestyle habits that increase their risk.

To investigate, Dr Siegel and colleagues used a computer program known as 'age-period-cohort modelling' to disentangle factors that influence all ages, such as changes in medical practice, from those that vary by generation, typically due to changes in behavior.

They analysed 490,305 cases among patients aged 20 and older across the US who were diagnosed with invasive bowel cancer between 1974 and 2013.

The study found that there was a slight dip in diagnosis rates after 1974.

However, from the mid-1980s onward, colon cancer incidence rates increased by 1 percent to 2 percent per year in adults aged 20 to 39.

Rectal cancer incidence rates have been increasing even faster, rising about 3 percent per year from 1974 to 2013 among 20- to 29-year-olds.

In contrast, rectal cancer rates in adults age 55 and older have generally been declining for at least 40 years.

Today, people in their early 50s are half as likely to develop colon and rectal cancer than that age group in the early 1990s.

This disparity is likely down to targeted testing and treatment of this age group - while the increase in risk among young people has crept up without much attention.

The findings follow another US study published in 2014 that predicted bowel cancer cases in 20- to 34-year-olds will rocket by up to 90 percent by 2030.