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%.