Supermarkets and manufacturers must be more open about so-called "shrinkflation" and "skimpflation", consumer group Which? says.
Which? said it was inundated when it asked shoppers for examples of products which were smaller or had fewer key ingredients.
It included boxes of tea bags and sausages with less pork.
The British Retail Consortium says retailers are trying to limit rising prices as production costs increase.
Which? picked out the following examples of shrinkflation:
Listerine Fresh Burst mouthwash shrank from 600ml to 500ml. At Tesco it also went up in price by 52p
PG Tips Tasty Decaf Pyramid tea bags went from containing 180 bags to 140 at a number of supermarkets
Kettle Chips Sea Salt and Crushed Black Peppercorns Crisps shrank from 150g to 130g at Tesco
Lurpak Slightly Salted Butter reduced from 225g to 180g at Morrisons and Sainsbury's
Yeo Valley Organic Salted Spreadable went from 500g to 400g at Sainsbury's and Tesco
It also found recipes for other products were altered to include fewer expensive ingredients:
In Sainsbury's 'Clotted Cream Rice Pudding', the clotted cream was replaced with whipping cream in its entirety. It has since been repackaged
Morrisons 150g guacamole lost some avocado, from 80% of its recipe to 77%
The amount of beef in a 1.5kg Tesco Beef Lasagne dropped from 23% to 19%
Waitrose Butter Chicken Curry lost some of its chicken content from 47% to 41%
Which? acknowledges that the changes have all come at a time when food inflation has soared. According to their food inflation tracker, year-on-year price rises peaked last April at 17%.
The consumer group said the research showed shoppers are often paying more for less.
"Supermarkets and manufacturers must be more upfront by making sure changes to popular products are clear, and by ensuring that unit pricing is prominent, legible and consistent in-store and online so that shoppers can easily compare prices across different brands and pack sizes," retail editor for Which?, Ele Clark said.
Director of food and sustainability at the British Retail Consortium, Andrew Opie, said nearly all of the examples identified by Which? reflected decisions by the manufacturers, rather than the retailers.
"Nonetheless, given the challenges facing households from the cost of living squeeze, retailers are solely focused to find ways to limit rising prices for customers against the rising cost of production, while maintaining the excellent quality of products," Mr Opie said.
"Prices and sizes of all products are clearly labelled so that customers can make informed decisions about their purchases."
