Shoppers who think they save money by buying in bulk may actually be paying more, according to an investigation exposing a variety of "misleading" supermarket offers.

Many items in shop multibuy promotions, such as "two packs of Innocent smoothies for £5", were actually cheaper if bought singly, an analysis of thousands of prices by a consumer group found.

In an exhaustive check of supermarket offers, Which? found several deals which looked attractive at first glance actually end up hitting consumers in the pocket.

Examples include raising the price of an item for only a few days shortly before putting it "on sale" for months, and advertising a higher "before" price than the one prior to the discount period beginning, both of which give the impression of greater savings.

The supermarkets said they had made honest mistakes and apologised to customers.

Which? tracked 700,000 prices over a year from 31 January 2011 and 1 February 2012 at the shopping website Mysupermarket.co.uk.

The consumer group did not quantify how many offers it considered to be potentially misleading because there were "so many", but highlighted 24.

Thirteen were at Asda, six at Tesco, two at Sainsbury, two at Morrisons and one at Ocado.

Among them, Tesco raised the price of a pack of blueberries from £1.80 to £3.99 for a fortnight before offering it for 52 days at £1.99 – still 19p more than before it was increased. Asda marked down Aquafresh milk teeth toothpaste to between £1 and £1.17 with a "was" price of £1.74, but Which? could only find it as having been sold previously at £1.17.

Among examples of dubious multibuy offers, Asda doubled the price of a single Müller yogurt from 30p to 61p; the price went back down to 30p when the offer ended.

And Tesco put wine on promotion for up to three months longer than it was sold at the advertised "was" price.

Which? called for a tightening of government guidelines on pricing, saying there were too many loopholes for retailers to exploit. The guidelines state that the "was" price should be the most recent a product was sold at for 28 consecutive days, and that a product should not usually be on offer for longer than it was at the higher price.

Richard Lloyd, Which? executive director, said: "At a time when household budgets are squeezed and food bills are going up, many people are on the look out for a bargain. It's unacceptable that shoppers are confused into thinking they're getting a good deal when that might not be the case. Consumers shouldn't have to worry about whether a special offer is really 'special'."

Most of the supermarkets said the offers highlighted were occasional lapses, though Morrisons disputed Which?'s findings.

Asda said: "We are only human and occasionally we make mistakes. By and large our systems and procedures ensure those instances are kept to an absolute minimum."

Tesco explained that it changed millions of price labels in store and online each week "and we sometimes make mistakes, for which we apologise".

Morrisons said the lower prices on multibuy items quoted by Which? were promotional discounts, emphasising: "The multibuy price is always cheaper per unit than the standard price."

Sainsbury said it had "accidentally" put the wrong "was" price on packs of The Black Farmer sausages but said the other offer highlighted by Which? – selling asparagus at higher price for only a fortnight before discounting it for two months – was within its guidelines.

Ocado regretted its "one isolated" example – a sharp increase in the price of pack of strawberries for 13 days before it went on offer.

Real deals? Offers that may save less than you think

Asda

Aquafresh Milk Teeth Toothpaste 0-3 Years (50ml)

Was: £1.74

Offer price: £1/£1.16/£1.17

Days on offer: 35

Highest price during Which?'s tracking was £1.18

Asda

Pedigree Chunks in Gravy Selection (12x400g)

Was: £7.21

Offer price: £6/£5

Days on offer: 77

Highest price during Which?'s tracking was £6.25

Products that were not sold at the 'was' price immediately before offer

Morrisons

Innocent Pure Fruit Smoothie Mangoes & Passion Fruits (750ml)

Price each before multibuy: £2 promo discount

Price each during multibuy: £2.79

Multibuy offer: two for £5. The multibuy offer makes the product more expensive per carton than it was before the offer.

Tesco

L'Oréal Elvive Colour Protect UV Filter Conditioner (250ml)

Price each before multibuy: £2

Price each during multibuy: £2.25

Multibuy offer: 3 for 2. It looks like the saving is 75p per bottle, but it is really only 50p per bottle.

Sainsbury's

Asparagus (250g)

Was: £3

Offer price: £1.50

Days on offer: 63

Only spent 14 days at the higher price before the offer was introduced, with a sign explaining the offer.

Asda

Philadelphia Original (300g)

Was: £2.37

Offer prices: £1, £1.77, £1.97, £2.12

Days on offer: 73