A survey has revealed that four in 10 people (41%) are unaware that a poor diet is linked to an increased risk of cancer.
Out of 2,000 people questioned by the World Cancer Research Fund (WCRF), only one in three knew that eating processed meat increased their cancer risk.
Meanwhile, nearly a quarter did not realise that being overweight heightened their chances of getting the disease.
Being overweight is linked to six types of cancer and 42% of people knew that not taking enough exercise increased their risk of developing one of them.
After giving up smoking, maintaining a healthy weight is the most important thing a person can do to hold off cancer, according to experts.
Lisa Cooney, Head of Education for the WCRF, said: "The scientific evidence is clear that people can reduce their risk of cancer through eating healthily and reducing their intake of things like alcohol and processed meat.
"But these results show that there are still millions of people in Britain who are unaware of how they can take simple steps to reduce their cancer risk.
"This means they are not in a position to make informed lifestyle choices."