The organisation recently shared guidance online explaining how shoppers can interpret codes used by major retailers to manage promotional pricing and stock rotation.

Tesco uses a reverse-style coding system on some promotional labels, which can make the expiry date of offers difficult to identify at first glance.

A Which? spokesman said: “There is a secret code which tells you when something is going to end on promotion at Tesco.

“They put it backwards so it can be a little bit tricky to spot.”

“Each code typically works by using the first number to represent the year, the middle number to show the month, and the final two digits to indicate the date the promotion will end.

“That means you can see something cheap, buy one to try, and know how long you’ve got to stock up in bulk if you like it.”

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Consumer groups said understanding these label systems can help shoppers better time purchases, particularly when planning larger household shops or bulk purchases of non-perishable goods.

Sainsbury’s uses a different system that shows when promotions began rather than when they will end.

Retail experts said Sainsbury’s promotional cycles often end on a Tuesday, meaning shoppers can estimate when discounts may expire even when an end date is not clearly printed.

Waitrose is generally more transparent in displaying promotion dates by listing both the day and month on shelf labels.

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Consumer feedback suggests many shoppers were previously unaware of the meaning behind label codes used across the supermarket sector.

Social media users responding to Which? posts said they had never noticed the codes despite regularly shopping at major supermarket chains.

Fresh produce also uses internal coding systems that are primarily designed to help staff manage stock rotation but can also be understood by customers.

In 2023, several supermarkets removed use-by and sell-by dates from some packaged fruit and vegetables as part of wider efforts to reduce food waste.

Despite this change, many items still carry letter and number combinations which indicate production or packing dates.

One Tesco shopper posting online said she noticed labels showing codes such as A11 and A12 on produce items, including potatoes and cucumbers.

The shopper said: “So the letter is the month and then the number is obviously the date.”

Under commonly used systems, the letter A represents January, B represents February, C represents March, with subsequent letters continuing through the year.

Retail experts said these codes can help shoppers identify which items are likely to last longer, although supermarkets still advise customers to follow storage guidance and check food quality before consumption.

Some shoppers noted that reduced price items often have shorter remaining shelf life, although others suggested freezing suitable items to extend usability and reduce food waste.

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Industry analysts said retailers use these systems primarily for operational efficiency rather than for customer use, although the information is visible to shoppers who know how to interpret it.

Which? said clearer labelling standards could help improve transparency for shoppers, although no industry-wide changes have been announced.

Retailers said customers can also check promotional terms through loyalty apps, in-store displays and online product listings.

Consumer experts said shoppers should always check label details carefully when buying discounted products to ensure they understand how long offers will remain available.

The guidance comes as households continue to monitor grocery spending closely amid wider cost of living pressures affecting food and household budgets across the country.