People are confused over whether screening programmes for bowel, breast and cervical cancer are to spot the disease or prevent it, research has found.
A poll of almost 1,500 people found many were unsure about the main aims of the screening programmes run by the NHS.
Carried out by the Cancer Research UK Health Behaviour Research Centre, based at University College London, the study found only 19% of women knew the main aim of cervical screening was to prevent the disease.
Meanwhile, 70% of men and women incorrectly thought that flexible sigmoidoscopy - where a long tube is used to look at the bowel - was primarily for early detection of bowel cancer.
In fact, its main aim is to prevent the disease by picking up changes - growths called polyps - that could eventually lead to bowel cancer.
Lead researcher Dr Jo Waller said: "The decision to take part in the screening programmes should be an individual choice.
"But in order to make a well-informed decision you need a good understanding of what's involved, the balance of benefits and harms, and also why you're being asked to take part in the first place.
"We're not sure why there's uncertainty about what these tests are for. So it's crucial we work with the public to make sure we're providing the best possible information and understand the common questions and misconceptions they might have."
Harpal Kumar, Cancer Research UK's chief executive, said: "We often talk about the importance of spotting cancer early because treatment is more effective and that gives patients the best chances of beating the disease. But, in many cases, cancer screening can also prevent the disease.
"The three national screening programmes have already saved thousands of lives since they were introduced. But we also know that screening carries risks as well as the benefits. It's absolutely essential that people are able to make an informed choice about taking part in screening and that means understanding what these programmes are designed to do."
Among women polled, 22% incorrectly thought the primary aim of breast screening was to prevent the disease.
In fact, it is to pick up breast cancer in its early stages.
Dr Anne Mackie, director of screening at Public Health England (PHE), said: "Deciding whether to accept a screening offer can be complex. Some screening programmes, such as breast screening, aim to find disease early and offer treatments to prolong healthy life.
"Other screening programmes (cervical screening) aim to detect problems before they develop into a disease and by treating them prevent the disease from developing at all.
"We do our best to make that difference clear in the leaflets and information we give people - and research such as this helps to guide us as we continually try to improve our information."