Many supermarket multibuy deals that promise savings on items such as pizzas, yoghurts, crisps and baby goods can leave shoppers worse off, according to a year-long investigation by Which? into "misleading" pricing tactics by the major chains.

In the worst cases, Which? found that supermarkets doubled the shelf price of an item when they began promoting it as a money-saving multibuy. It found that Asda was selling a Goodfella's Deep Pan Pepperoni pizza at a standard price of about £1, but when it went on to a multibuy deal, the price jumped to £2.50 for one or £4.50 for two.

Around one in 10 products on supermarket shelves increased in price when they went from the previous standard price to the new multibuy price, then decreased again when the promotion ended.

Which? also found that some products were almost permanently on discount or multibuy. It cited a Nestlé yoghurt, Munch Bunch, which was typically on sale for £1 for a pack of four at Tesco and Sainsbury's. But when it went on multibuy, the price was £2 for two packs, or £1.40 to £1.59 for one pack. "So whenever you buy, the product nearly always costs £1," said Which?.

At Asda, a 200ml bottle of Ambre Solaire sun protection cream was usually on sale for £5 when not on multibuy, but then £7 for one or £10 for two when promoted as a multibuy. "Generally, it was the same price whether on offer or not," said Which?.

Waitrose also fell foul of the price-checking exercise. Over a period of four months, it sold six-packs of McCoy's crisps on a multibuy of £2.50 for two, or at a discount price of either £1 or £1.26.

Asda told Which?: "Customers tell us they want low prices, not a strategy that gives with one hand and takes with the other. We're making sure the cost of the weekly shop is consistently low, with no surprises."

A spokeswoman for Sainsbury's said: "Our stores have around 30,000 products and Which? has cited just one example. We run a wide variety of promotions to meet our customers' needs and we always clearly display the standard price with multibuys to help prevent any confusion."

Tesco declined to comment on the specific findings of the investigation but issued a statement saying that its multibuys offered good value for money.

Some of the deals found during the investigation were "plain daft", said the consumer group. "We went around supermarkets and found packets of sweets that were 34p each or four for £3," said Which?. The Guardian's Saturday Money section has in recent months featured dozens of "daft deals" submitted by shoppers in which the multibuy is pricier than buying the items individually.

Multibuy deals have become a staple of supermarket promotions in recent years, and now apply to nearly half the goods on offer in a supermarket at any one time. Which? said that of the 115 products that it examined in the first half of this year, 46% of the time they were on multibuy, compared with 35% of the time in the first half of 2011. But there are signs that consumers have become increasingly disillusioned by the value on offer in supermarket promotions. Seven out of 10 people told Which? that they prefer straightforward discounts to multibuy offers.

The Office of Fair Trading said it could not comment on individual supermarket prices, but that its research had found most shoppers automatically assumed multibuy offers were cheaper than buying individually, and that 37% did not work out if they were actually making a saving when they bought the item.

The shadow consumer minister, Ian Murray, said: "Consumers in the UK deserve complete clarity on best buys and discounts from our major supermarket retailers, particularly due to the fact the pay packets aren't stretching as far as they previously did, so every penny counts.

"There is a strong case to be made that the government should look into this area now the evidence of Which? has come to light. While competition among supermarkets is good for consumers, and indeed they make a major contribution to our economy, there is a risk that people, particularly families who may be reliant on these sorts of multibuys, are being ripped off."

Which? said existing rules were supposed to make sure that supermarket special offers did not mislead shoppers, but it felt they left too many loopholes and would call for the government to improve consumer protection.