Tesco customers are being short-changed by promotions which have expired but are still advertised on the shelves, a BBC investigation has found.
It revealed shoppers were overcharged at two-thirds of stores visited by an undercover reporter.
In 33 of 50 stores visited, multi-buy promotions were marked on the shelf, but the time-limited discounts were not applied at tills.
Tesco said it would double-check the accuracy of pricing at all its stores.
A reporter for BBC's Inside Out visited the stores over a three-month period.
He found multi-buy deals still on display days, weeks and - in some cases - months after the deductions were no longer being applied at the till.
Mistakes only applied to in-store offers, and not those online.
The investigation was triggered when discrepancies in receipts were discovered by a member of BBC staff.
On one occasion, the reporter told customer service staff at a store in Dudley offers for gingerbread and cat food had not gone through.
Examples of how the BBC was overcharged
- Rustlers Chicken Sub: Offer price - Two for £3 (original price £2.31 each) Price paid at till - £4.62
- Schwartz Barbecue Sauce Mix: Offer price - Two for £1 (original price 85p each) Price paid at till - £1.70
- Old El Paso Guacamole: Offer price - Two for £2.50 (original price £2.09 each) Price paid at till - £4.18
The staff member was filmed replying: "The labels are actually out of date, that's what the problem is.
"One's nearly a month out of date, the other one's three weeks out of date."
At another store a member of staff blamed the expired offers on them being "short-staffed".
Analysis: Martin Fisher, Chartered Trading Standards Institute
There is no real idea of the level of the problem [in the wider supermarket industry] because it is not recorded in enough detail to be searchable.
The underlying problem is that people don't complain because they don't realise. Very few people remember [the price] from the shelf to the till, so complaints are extremely low.
I would really hope that Tesco are out there on their own on this. I would hope that other stores have better systems to check and feed back and learn from their customer complaints.
In every case where the discrepancy was brought to light, Tesco staff honoured the offer.
But in the worst example, staff at a Tesco Express outlet in Birmingham repeatedly failed to remove labels showing produce as on offer when it no longer was.
It was still on display a month after the reporter first pointed it out.
Martin Fisher from the Chartered Trading Standards Institute said the mistakes could amount to breaking the law, which falls under The Consumer Protection from Unfair Trading Regulations 2008.
Tesco said it would be double checking the accuracy of the price labels at all its stores as a result of the BBC's investigation. The company runs more than 3,500 stores across the UK.
A spokesperson said: "We take great care to deliver clear and accurate price labels for our customers so they can make informed decisions on the products they buy.
"We are disappointed that errors occurred and will be working with the stores involved to reinforce our responsibilities to our customers."
*Additional reporting by Lisa Wright
The full investigation can be seen on BBC Inside Out in most English regions at 19:30 GMT on BBC One on Monday 13 February and for 30 days after on the BBC iPlayer