All the supermarkets on the net, at the click of a button

As prices continue to rise, Laura Harding looks at a site that claims to take the strain out of shopping around and help families save £900 a year

At a time when the price of an average weekly shop is increasing and consumers are having to dig deeper into their pockets when they reach the checkout, getting the best value for money is more important than ever.

Between January 2007 and January 2008, food bills rose by 7.4 per cent, adding £750 to the average family's annual shopping bill, the Office for National Statistics has reported.

While comparing the prices on offer from different stores remains important, trawling the supermarket aisles looking for the best bargains and then lugging your heavy shopping bags to the car could soon be a thing of the past, thanks to the internet.

The comparison website mySupermarket.co.uk lists every product available online from Tesco, Sainsbury's, Asda and Ocado (the internet arm of Waitrose), although not from chains that do not offer a delivery service, such as Morrisons. It lets you choose your groceries from your favourite supermarket, flags up any special offers and then scans your trolley contents and suggests swapping some items for others that are better value or have a lower calorie count. It also shows you how much your trolley would cost at another supermarket. You can then "send" your trolley to the online checkout at whichever retailer works out cheapest overall, and have the groceries delivered to your door.

The website claims that a family spending £90 a week would make an average annual saving of £936 by doing all their shopping at the best-value store.

Martin Lewis from the financial advice site MoneySavingExpert.com praises mySupermarket.co.uk for making shoppers think about the best ways to save money: "It's such a powerful tool that, even if you don't want to shop online, it's a good way to get an idea of how much your shop will cost in the different stores."

Johnny Stern, director of mySupermarket.co.uk, said the most important feature of the website was the price-checker button, which he claims could save a shopper around £20 a week on like-for-like products: "A lot of consumers are loyal to one supermarket, and every week they have their list and know where they are going to shop, so the price-checker button allows them to search for cheaper like-for-like goods [in other supermarkets] that they would find it impossible to look for themselves.

"If an average shop is around £100, we would expect them to save about 20 per cent by swapping to other products from the same supermarket. When the website tells you if your basket would be cheaper elsewhere and you decide to swap to a different supermarket, you would save another 10-20 per cent."

A spokesman for Tesco has warned that the choice in the mySupermarket baskets might be somewhat narrow because the website scans only branded products. However, this claim is strongly denied by Mr Stern. Indeed, a quick trawl shows a wide variety of own-brand goods listed.

In turn, Mr Stern hits back at the big chains by advising consumers to beware of superficial special offers that don't provide genuine savings: "Supermarkets constantly highlight that they are reducing prices but they are not looking to reduce the amount you spend. When they reduce the price of one thing, they put it up on another to compensate."

However, Nicola Frame, a spokesperson for the consumer group Which?, said shoppers should use price comparison sites such as mySupermarket.co.uk with caution. "They are a good starting point but they often don't include all the products you are likely to need. So make sure you look at other websites for supplementary information.

"[Supermarket comparison sites] are certainly a useful tool, but when they don't list everything, that can make matters confusing. Whatever you are looking to buy, be it groceries or insurance, there is an overwhelming choice, and websites can do a lot of the legwork for you, but you should be careful."

A website that can be used in conjunction with mySupermarket.co.uk is fixtureferrets.co.uk, which searches for grocery promotions at all of the UK's main supermarkets and highlights value-for-money deals such as buy one get one free, price reductions, multisaves, linksaves and bonus points.