Which? said it continued to find evidence that commonly-bought items such as tea, cheese and washing powder had shrunk – and in some cases actually increased in price.
It found that Sainsbury’s increased the cost of Tetley Blend of Both tea from £2.55 to £2.75, despite the pack shrinking from 80 tea bags to 75.
Asda increased its price for Philadelphia Light Soft Cheese from £1.84 to £1.87 after the pack shrank 10% from 200g to 180g.
Surf Essential Oils Powder Tropical Lily & Ylang Ylang remained priced at £5 in Ocado and Tesco despite shrinking from 25 washes (2kg) to 23 washes (1.61kg).
And at Asda, Morrisons and Sainsbury’s, the prices of Birds Eye Select Mixed Vegetables and Hovis Best of Both bread remained the same despite shrinking.
The watchdog reported that the majority of manufacturers said it was retailers who ultimately set the prices of products.
But it pointed out that supermarkets were unlikely to drop prices unless the wholesale cost fell.
It said: “Products are losing 50g here and a few centimetres there, which is all adding up to a more expensive shop without you knowing why.”
Which? executive director Richard Lloyd said: “Shrinking products can be a sneaky way of putting up costs for consumers because pack sizes shrink but the prices don’t.
“It’s now time for action on dodgy pricing practices that stops people from easily comparing products to find the cheapest.”
Mondelez International, which owns the Philadelphia brand, said: “It’s a fact that we face rising costs across a number of areas, including ingredients, but at the same time we are aware consumers are looking to control their shopping spend.
“In order to be transparent with consumers, we clearly state the new weight on the front of all Philadelphia tubs.”
A Birds Eye spokesman said: “In March 2014 we introduced zip-lock packaging to our Field Fresh range. Consumer feedback told us that consumers would welcome this innovation, with findings showing they were willing to pay an increased price for the added benefit of re-sealable packaging.
“Having consulted consumers, we decided to keep the product price the same but reduce the total servings slightly, to allow for the increased cost of re-sealable packaging. The resulting response from our consumers has been very encouraging.”
A spokeswoman for Unilever, which owns the Surf brand, said it reduced the recommended retail price in line with the new pack size.
She said: “As a manufacturer we can only recommend the retail selling price. The in-store price is solely at the discretion of retailers.”
A spokeswoman for Hovis said: “The packaging change took place in 2013 and current Hovis Best of Both loaf prices vary significantly in retail stores. The change from a 800g to 750g format was the result of extensive consumer research and feedback and the small change in weight coupled with a recipe update represented a significant product improvement and health benefit to consumers.
“Along with the slight weight reduction, the reformulation means that a slice of Hovis Best of Both contains on average 20% less salt and calories than our closest competitor.”