Your supermarket shop cost less in September than at any other time this year - although you need to be more vigilant than ever to avoid falling for pricing gimmicks, a grocery comparison site has claimed.

Mysupermarket.co.uk analysed the cost of 35 weekly essentials at many of the major supermarkets, with the total cost coming in at £85.93 in September.

This is slightly cheaper than last month's bill of £86.75, and the lowest seen all year as heavy competition between rivals dragged down prices at the tills.

Indeed, aside from two months of unexpected rises over the summer, prices have been falling steadily for some time now. Research shows you would have paid 5%, or £4.88, more for the same shop last September.

While falling prices mean bargains are there to be had, the research noted that many supermarkets were dramatically changing price on a monthly basis in a bid to catch out shoppers.

For example, the price of pasta jumped almost 6% between August and September, while cucumbers (4%) and carrots (4%) were also notably more expensive.

Mysupermarket CEO Gilad Simhony also reminded shoppers to bring their own carrier bags from next week or risk adding to their shopping bills as the new 5p bag tax kicks in.

''Although we welcome the fall in grocery prices, the latest figures should be met with caution as they are part of an environment of confusing and regular price changes designed to catch shoppers out and pay more for the products they love.

"The arrival of carrier bag charges this month heralds another charge that consumers will be expected to cover. It is in this context that we advise shoppers to check prices as the savings can add up significantly.''

How the cost of 35 weekly essentials has changed:

December - £90.24

January - £88.13

February - £88.59

March - £87.70

April - £86.69

May - £86.60

June - £86.84

July - £86.11

August - £86.75

September - £85.93