Channel 4 Dispatches enlisted the help of mystery shoppers in 32 locations around the country to visit their local market or independent trader, a large supermarket and a branch of that supermarket's convenience store.
They were asked to purchase three everyday fruit and vegetable items - broccoli, pears and coriander - for the cheapest price available in each outlet.
When the prices they paid for these three items were analysed there was some interesting results:
When it came to individual items:
Tesco and Sainsbury's say its common practice for retailers to charge a small amount more for some products through their convenience store due to higher operating costs in running these stores.
Channel 4 Dispatches airing tonight at 8pm investigates how the supermarkets sell fresh fruit and veg, and how their prices compare with independent traders.
Reporter Tazeen Ahmad investigates the startling rise of supermarket convenience stores, and asks: are we paying a fair price for on-your-doorstep convenience, or are we being taken for a ride?
The supermarkets dominate the sale of both fruit and vegetable.,
Last year shoppers spent £81m a week on fruit - with £69m (85%) of that went through the supermarket check outs while independent outlets took the remaining £12m.
Shoppers spent even more on veg - £105m each week - of which £88m (84%) was spent in the supermarkets. Independent outlets took the remaining £17M (16%).
The rise of the Supermarket Convenience Store
One of the biggest changes on the High Street has been the growing importance of convenience store shopping.
69% of adults now visit one of these every week and they now account for more than 20% of all food spending.
Sainsbury's now have 487 of their Local stores. But Tesco lead the way with nearly three times that number (1487).
Sales in convenience stores are growing fast. They are being heavily promoted and new ones are springing up all the time.
So what is it that really makes the supermarket convenience stores so popular?
With more people living and working longer hours and, and living in a different way, and eating out of the home more the super- the small, convenience store has actually become a major outlet for very large numbers of population.
Calls for more transparency on supermarket pricing
There's growing unease at Westminster about the pricing techniques of some supermarket convenience stores.
It was the subject of a debate only last week.
John Denham MP thinks he has the answer to the problem - forcing supermarkets to hand over all their price information.
"I want supermarkets to make available all the data they have on the price of their goods, product by product, store by store. They already have it internally on their computers but if they make it available that will enable consumers through smart phones and other devices to compare prices of different products in different stores," he says.
"It's very difficult for consumers to know how much more they're being charged for the convenience or the necessity of using a local store when the big store's some time away.... That's the sort of thing you ought to be able to work out on your smartphone and you could actually work out whether it's worth getting on the bus to go to the cheaper supermarket", he adds.
Secret's of Your Supermarket Shop - Channel 4 Dispatches, Monday 21st January, 8pm