The research by Newcastle University and the University of Leeds found that being overweight more than doubled the risk of bowel cancer in people with Lynch Syndrome.
The genetic disorder affects genes responsible for detecting and repairing damage in the DNA and around half the people with it will develop cancer.
However, the 10-year trial found the risk could be counteracted by taking a regular dose of aspirin.
The international study, published in the Journal of Clinical Oncology, covered 43 centres in 16 countries and followed nearly 1,000 patients with with syndrome.
John Burn, professor of Clinical Genetics at Newcastle University, said: “This is important for people with Lynch Syndrome, but affects the rest of us too. Lots of people struggle with their weight and this suggests the extra cancer risk can be cancelled by taking an aspirin.
“This research adds to the growing body of evidence which links an increased inflammatory process to an increased risk of cancer.
“Obesity increases the inflammatory response. One explanation for our findings is that the aspirin may suppress that inflammation.”
A large-scale follow-up trial is now expected with 3,000 people across the world testing the effects of different doses of aspirin.
The trial will compare two aspirin a day with a range of lower doses to see if the protection offered is the same.
Prof Burn said: “Before anyone begins taking aspirin on a regular basis, they should consult their doctor as aspirin is known to bring with it a risk of stomach complaints including ulcers.
“But if there is a strong family history of cancer then people may want to weigh up the cost-benefits, particularly as these days drugs which block acid production in the stomach are available over the counter.”