Tesco is to check the prices of all items in every store after an investigation found customers were being short-changed on promotions.

An undercover reporter for BBC Inside Out was overcharged on multibuy offers at two-thirds of stores visited.

The reporter was charged more than expected in 33 out of 50 stores because marked promotions were out of date and no longer valid at the tills.

In some cases, multibuy deals were still being advertised on the shelves months after they had expired.

Martin Fisher, from the Chartered Trading Standards Institute, said such errors could break consumer protection laws under Unfair Trading Regulations 2008, the BBC said.

The supermarket giant says it will now re-evaluate the price of every item at its 3,500-plus stores across the UK.

Tesco said: “We are disappointed that errors occurred and will be working with the stores involved to reinforce our responsibilities to our customers.”

• The full investigation will be broadcast in most English regions at 7.30pm on BBC on Monday.