Just one in five Britons eats the recommended five portions of fruit and vegetables a day, a poll for World Cancer Research Fund (WCRF) suggests.
The Department of Health first launched its five-a-day campaign in 2003.
But the WCRF says its survey of more than 2,000 UK adults shows people still find achieving that goal difficult.
It is urging people to eat "just one more portion" for a healthier diet, which would increase cancer protection.
The YouGov survey for the WCRF found that, on average, 17% of lower income households (social groups C2, D and E) eat at least five portions per day, compared with 27% for those in higher income groups.
Fruit and veg consumption levels were lowest in the north of England, where 18% had five or more portions daily.
The highest levels of consumption were reported in the south of England, where 26% said they ate at least five portions.
In London, it was 21%, in Scotland 22% and in Wales 23%.
Kate Mendoza, head of education for the WCRF, said: "These figures show that many people are still finding it difficult to follow the healthy eating message.
"Getting at least five portions of fruit and vegetables a day is the building block of a healthy diet. Not only are fruit and veg a good source of nutrients, they also tend to be low in calories and full of fibre and so help us maintain a healthy weight.
"A diet based on plant foods, such as wholegrains and pulses as well as fruit and vegetables, can reduce cancer risk as research shows they protect against a range of cancers. Recent research has confirmed that foods containing fibre reduce the risk of bowel cancer.
"Although people are more aware of the significance of eating five a day than they used to be, it is clear that there are still barriers to incorporating plant foods into our daily diets," she added.
The charity is holding an awareness day, "Fruity Friday", on 18 May as part of Cancer Prevention Week.
A Department of Health spokesperson said: "We know we need to do more to encourage people to eat their five a day and help prevent diseases like cancer.
"That is why, through our Change4Life campaign, we invested around £10m last year on encouraging healthier lifestyles. This included things like the Supermeals campaign which promoted five a day by giving recipe ideas and money off fruit and vegetables in retail stores."