Some big name brands are shrinking products by up to a quarter while keeping or even increasing the original price, a study has found.
Among the everyday products identified by the Which? consumer group were a pack of Birds Eye beef burgers with four fewer burgers than the original 16 despite the price increasing from £3.98 to £4.29 in Asda, Morrisons and Tesco.
Pledge furniture polish has shrunk by 17% but the price has stayed the same or risen, Nestle Munchies dropped from 150g to 126g with a disproportionate price drop from £1.67 in Asda and Sainsbury's to £1.59, and Thorntons Mini Caramel Shortcakes increased in cost by 10p in Waitrose despite shrinking from 12 cakes to 10.
The study found "anything from bags of crisps to packs of dishwasher tablets are getting smaller".
The watchdog said it was generally told by the product makers that they chose to shrink products rather than increase prices to cope with rising costs.
However, a poll of consumers by Which? found 58% would prefer prices rose than packs got smaller, while 37% would rather the pack shrank but only if they were informed. Just 3% said it was acceptable for the pack size to shrink and not be told about it.
Which? added that many of the manufacturers it spoke to said retailers ultimately set prices, but when asked if they reduced the wholesale price or set a lower recommended retail price to match shrinking products they either said they did not do this or would not comment.
Nestle said it had dropped the cost price but refused to say if it was proportionate to the amount its products shrank by, while Thorntons said shrinking allowed for better promotions.
Which? executive director Richard Lloyd said: "Shrinking products can be an underhand way of raising prices because pack sizes shrink but the prices don't. We want simpler pricing so people can easily compare products to see which is the cheapest, and for special offers to be genuine."
Which? surveyed 1,257 members in January.