Hundreds of supermarket products have shrunk in size but prices have stayed the same or increased, according to new research revealed today.

Consumer group Which? claimed grocers were being “sneaky” by selling basket staples including toilet rolls, toothpaste, orange juice and biscuits at the same price, despite decreasing pack sizes.

The watchdog said Andrex cut the number of sheets on a standard toilet roll by 8%, from 240 to 221, but prices had remained constant at around £2 for four rolls.

In another example, Which? singled out Tesco for allegedly increasing the price of McVitie’s dark chocolate digestives from £1.59 to £1.69, even though the pack size had dropped more than 10% from 332g to 300g.

Its big-four rival Asda is said to have kept the price of Tropicana Creations Pure Premium orange and raspberry juice constant at £2.48, despite a reduction in carton size from 1 litre to 850ml.

In another example, Which? claimed that although Morrisons dropped the price of Sensodyne toothpaste from £4 to £3.49 after it decreased in size, shoppers were still paying more per 100g than they were before the pack shrunk.

Dettol bathroom wipes reduced in size from packs of 36 to 32, but the price remained at £2 in Tesco and actually increased slightly to £2.03 at online grocer Ocado, Which? claimed.

Which? editor Richard Headland said: “Shrinking products can be a sneaky way of increasing prices. We want manufacturers and supermarkets to be upfront so consumers aren’t misled.”

A poll by Which? found that 78% of its members did not think it was right if consumers were not told when products dropped in size but remained at the original price.