Most people are increasing their chances of developing heart disease, cancer and other chronic conditions later in life by failing to eat the recommended five portions of fruit and vegetables a day, research shows.

A majority of the 1,347 people who took part in the internet survey last month were confused about the "five-a-day" message, with many wrongly believing a portion of fruit and veg could be orange squash, herbs or chips.

Two-thirds of people who took part in the poll – five years after the Government launched the "five-a-day" campaign – confessed to feeling guilty they had eaten badly. The findings correspond with Food Standards Agency research published in February which found that only 58 per cent of adults were hitting, or believed they were hitting, the target.

"It's incredible there are still so many 'five-a-day' myths in existence. This is worrying because achieving the target is an important component of a healthy lifestyle and reducing risk of diseases such as heart disease and cancer," said Dr Sarah Schenker, a nutritionist and British Nutrition Foundation member.

Under the Department of Health scheme launched in 2003, adults are urged to eat five 80g portions of fruit and vegetables a day.

A portion is one apple, banana or orange; one slice of a large fruit such as melon or pineapple; or three tablespoons of a vegetable. Fruit juice counts as a portion but potatoes are excluded.

According to the poll for Tropicana Smoothies, 66 per cent of adults claim to understand the "five-a-day" mantra, but many failed to achieve the target because they had "misinformed ideas". Some 60 per cent of people believed five pieces of fruit alone ticked the "five-a-day" box, while the Government specified there should be a mix of portions.

A quarter of respondents thought orange squash counted, while many were similarly misguided about herbs (10 per cent) and chips (3 per cent).

The British Dietetic Association said there had been only modest changes in the diet since the "five-a-day" scheme's launch.

"The average is three servings a day," said a spokeswoman, Ursula Arens. "Women are having about the same as men, but if you look at the calorie intake women are doing better than men. Older people are doing better than younger people."

The registered dietitian added: "Generally, our diets are too high in fat, too high in sugar, too high in salt and too low in fibre, and not achieving the fruit and veg every day. Although intakes of calories are not much higher than in the past, because our lifestyles are so sedentary, we have obesity. We know from lots of epidemiology that diets that are high in fruit and vegetables seem to be associated with lower rates of heart disease and some cancers."

Cancer and coronary heart disease account for 60 per cent of all early deaths and a "key feature" of the Government's strategy to reduce early deaths from the diseases is improved diet and nutrition.

Todd Katz, of Tropicana Smoothies, said: "We were surprised to learn that "five-a-day" is still widely misunderstood, despite the term being commonly known."