People worry unnecessarily about cancer in their family because they do not realise only a small number of cancers are hereditary, a survey suggests.
Most cancers are not passed on in the genes
The poll found 91% of more than 1,000 people polled falsely believed that if a relative has had cancer, they are at a greater personal risk of the disease.
In fact, the likelihood of an increased risk is small, as nine out of 10 cancers appear by chance.
The survey was carried out by information charity Cancerbackup.
The poll, conducted through the Genes Reunited website, found a quarter of people thought that between 50% and 100% of cancers are hereditary.
The majority of people (74%) wrongly thought that if several members of their family have had different types of cancer, it means that there is a strong chance of an inherited genetic link in the family.
It also found that 60% of people think family history is the biggest risk factor for cancer - only 15% realise that it is actually age.
Two-thirds of cancer occurs in people over 65 years old.
Dr Andrea Pithers, Cancerbackup genetic information manager, said: "Worrying about cancer can be very debilitating and it is sad that people are not aware of the reality, that very few cancers are caused by a known inherited genetic link.
"At the same time people should know about how they can lower their risk with things they can actually change, like eating a healthy diet and exercise."
Martin Ledwick, information nurse manager at Cancer Research UK ,said the results showed how important it was that people got accurate information about cancer risks.
He said: "While most cancers are not strongly linked to family history, if people are worried they should speak to their GP for advice.
"Half of all cancers could be prevented by changes to lifestyle."