A new study suggests that seven, rather than five, portions of fruit and vegetables every day should be the target for those looking to live as long and healthy a life as possible.

Researchers from University College London used data taken from the National Health Survey between 2001 and 2008 to look at the fruit and vegetable intake of 65,226 men and women.

Lead investigator Dr Oyinlola Oyebode told the BBC: "The clear message here is that the more fruit and vegetables you eat, the less likely you are to die - at any age."

Risk of death by any cause was reduced by 14% by eating between one to three portions of fruit and vegetables per day, 29% for three to five portions, 36% for five to seven, and 42% for seven to ten.

The best foods are fresh vegetables, followed by salads and dried fruit. The study found that fruit juices have almost no benefit and canned fruit appeared to actively increase the risk of death by up to 17%, possibly because of the sugary syrups the fruits are often preserved in.

Despite the large sample size, the study has been criticised due to lack of proper data control. Professor Tom Sanders from Kings College London's School of Medicine said to the BBC that it was "already known" that people who claimed to eat lots of fruit and vegetables were also likely to be wealthier, more educated and more health conscious.

"You cannot extrapolate from this kind of information to make sensible pronouncements about what people should eat," Professor Saunders claimed.

The five 80g portions per day government advice began in 2003 and was based on recommendations from the World Health Organisation. France and Germany also have their own five-a-day campaigns, while the US runs a less specific campaign called 'fruit and veggies – more matters'.

The country with the advice most praised by the UCL researchers is Australia, where they have a campaign advising citizens to 'go for 2+5'. This refers to two 150g portions of fruit and five 75g portions of vegetables.

Dr Oyebode praised the Australian campaign in the Guardian saying: "I think it makes a lot of sense. It is aiming for more and the balance is two fruit and five veg. From our study it looks like vegetables are better than fruit."

However she does not feel there is a need to change the UK Government's current advice on fruit and vegetable intake: "I don't feel very strongly that the guidelines should be changed because the majority of people know they should eat five a day and only 25% manage that."

Professor Naveed Sattar, of the University of Glasgow agreed with Dr Oyebode's concern over the ability to further enhance the country's eating habits by increasing the advised intake levels.

Professor Sattar said to the BBC that the idea of advising people to eat seven a day was a "really challenging" prospect.

"It would require governmental support such as subsidising the cost of fruit and vegetables, perhaps by taxing sugar-rich foods, and making available high quality products to all in society."

Christian Today surveyed thirty one products across three different supermarket chains in central London, looking at various products ranging from salads, soups, fresh fruit and vegetables, all of which were advertised as being part of the five a day. The average cost of one portion was £1.42, making seven a day cost £9.92. Based on this average, one week's worth of fruit and veg would come to around £69.44.

Some supermarkets did not have a clear pricing structure pegged to the five a day, with Sainsbury's selling a Thai vegetable curry which contained two of the recommended five a day for £2.50, while also selling a potato and egg salad with one of the five a day for the same price.

Loose fruit was the cheapest item, with the Co-Op selling loose oranges for 40p, an 80g bag of seedless grapes for 50p, and a punnet of 16 strawberries, of which 7 count as one of the five a day, for £2.

Vegetables are similarly priced, with Tesco providing one portion of tenderstem broccoli for approximately 72p, and one portion of baby carrots for 64p.

Prepared items such as salads and soups however are much more expensive. A Tesco peppery salad bowl costs £2.50, a Co-Op smoky tomato and pulled pork soup is £2.00, and a Sainsbury's cream lasaka noodle soup is £3. All of these were advertised as counting toward the five a day.

The manager of The Strand's branch of Sainsbury's Local, Raquel Hodregues, said that the five-a-day advice did affect people's buying habits: "The demand is mostly for the kids, for things like the boxed fruit salads and things like that."

Asked if she felt the prices were reasonable, she said: "You can get the same product in lots of different ranges, so the prices vary a lot and I would say they are reasonable. We are doing different promotions every week on the five-a-day items."

Jeremey Ross, a duty manager at the Bedford Street branch of Tesco, also believes the five-a-day rule is influencing what people put in their baskets.

"It's definitely affected people's buying habits, with people buying more veg. I know it's affected me, I love a good steak, but now I make sure I get some vegetables with it," he said. 

Asked what he thought of prices, Mr Ross said: "We're running different offers all the time. This week you can get a big tree of broccoli for a pound, so I'd say things are fair."

Dr Oyebode suggested that policy changes could be necessary to combat this kind of problem: "Anything that could increase the accessibility and affordability of fruit and vegetables would be very helpful, such as working with corner shops to make sure they stock them."

According to studies in America looking into so-called food deserts - areas where affordable and healthy food is difficult to obtain, especially without a car - simply making the healthier food more available is not enough.

Data from the journal 'Health Affairs' published in February found that of 1,000 Philadelphia residents who six months previously had been living in food deserts who now had healthy produce sold less than 1.5 miles from their home, only 26.7% are using the healthier suppliers.

The research also points out that within that 26.7% there has been no significant increase in health metrics since they began using the new supermarkets. Body mass indexes of the group studied remained simmilar to how they had been before the arrival of the new supermarkets

Dr Alison Tedstone, of Public Health England has said to the BBC that the UCL study has not resulted in any reconsideration of the UK government's five-a-day advice plan: "I think we should keep it simple and stay as we are.

"We are working very hard to improve the availability of fruit and vegetables, as we see it as absolutely integral to somebody's health to choose those five a day."