Four of the UK's biggest supermarkets could face prosecution over misleading pricing practices, a consumer law expert has warned.

In a report on supermarket price wars for BBC1's Panorama programme, Deborah Parry said many of the pricing tactics used by Tesco, Asda, Sainsbury's and Morrisons could potentially be illegal.

The report comes at the end of a year in which the big four have opened more than 200 new stores. Between them they now control 68% of the UK grocery market with £65 billion of the £96 billion spent in the last year going through their tills.

In the programme, Panorama reporter Sophie Raworth takes a trip around the big four supermarkets near her home and finds mistakes and misleading claims in all of them. She found 17 items presented as bigger pack, better value which were in fact worse value. Five were in Tesco, five in Sainsbury's, four in Asda and three in Morrisons.

The supermarkets said they display unit prices so shoppers can compare but admitted when smaller packs are on promotion they may sometimes be cheaper.

Ms Parry said: "It is not just the occasional mishap here. There are repeated examples with many, many products in different locations with different supermarkets and all of them seem to be doing the same thing. So there is a potential for prosecutions to be brought against all of them."

A survey of 1,546 shoppers carried out for the programme revealed 42% of shoppers don't trust supermarket offers and discounts. 31% said they were less likely to trust them now than in the past and 47% said they had felt misled by offers.

Leading retail analyst Richard Perks told Panorama any so-called price war between supermarkets is a marketing mirage. He said: "If there was a price war going on at the moment we would have seen profits falling, or we would have severe warnings and that is just not happening."

Last December, the Office of Fair Trading warned supermarkets to stop using misleading pricing practices, or face "enforcement action". But Panorama found promotions that offered zero savings - such as a product being advertised for £1, with two for £2.

All four of the big supermarkets deny misleading or deceiving customers. They say they work hard to keep prices down and point to recent research from the Office for National Statistics which attributed last month's fall in inflation in part to supermarket promotional campaigns.