HIGH levels of iron may be one reason why eating red meat raises the risk of bowel cancer.
Iron may interact with a faulty gene in the gut to trigger cancer, scientists said.
Red meat contains large amounts of iron and is also known to increase the likelihood of bowel cancer.
The discovery could lead to new cancer treatments that target iron in the bowel.
In studies of mice, researchers found that susceptibility to bowel cancer was strongly influenced both by iron and a gene called APC.
When the APC gene was faulty, mice with a high iron intake were two to three times more likely to develop the disease. Mice fed a low iron diet remained cancer free even if the gene was defective.
But when APC was normal, high iron levels did no harm.
Lead scientist Professor Owen Sansom, deputy director of the Cancer Research UK Institute, said: "We've made a huge step in understanding how bowel cancer develops.
"The APC gene is faulty in around eight out of 10 bowel cancers but until now we haven't known how this causes the disease.
"It's clear that iron is playing a critical role in controlling the development of bowel cancer in people with a faulty APC.
"And, intriguingly, our study shows that even very high levels of iron in the diet don't cause cancer by itself, but rely on the APC gene."
Without a working APC gene, iron is allowed to build up in the cells lining the gut, said the researchers writing in the journal Cell Reports.
This activates a genetic cancer "switch" called 'wnt' that causes cells to multiply out of control. Iron also had the effect of increasing numbers of cells with defective APC, by helping them to survive.
In mice fed a diet with no iron, cells with a faulty APC gene were killed off and bowel cancer did not develop.
Dr Julie Sharp, senior science information manager at Cancer Research UK, said: "Bowel cancer is the third most common cancer in the UK.
"These findings suggest a potentially effective way of reducing the chances of bowel cancer developing in people who are at high risk.
"Finding ways of 'mopping up' the iron that is in the bowel could have a real impact on the number of people who develop the disease."