Some tablet manufacturers are charging more than a 1,000 per cent mark-up for extra storage space.
The watchdog claims consumers opting for a 32GB iPad Air will have to pay £80 more than they would for the 16GB version, despite it costing Apple less than £6 to buy the additional memory at market prices.
It said tablet manufacturers can buy Flash memory storage at a market price of £5.95 for 16GB, yet Google charges an extra £70 to increase the memory on its Nexus 10 from 16GB to 32GB and Amazon charges £40 for the same storage increase on its Kindle Fire HDX 8.9.
It described the mark-ups as "outrageous" and advised potential customers to buy a tablet with an SD or microSD slot and add a memory card for a fraction of the cost.
The report said: "The best thing about using a tablet is filling it with apps, songs and videos that you love. But all of this content gobbles up storage space fast, and tablets use Flash memory that's limited in size.
"Unlike with PCs, tablet storage is difficult to upgrade after you've purchased your model. That's because most tablet bodies are sealed and don't let you install further hardware."
Which? also found that the true amount of storage space is not the same as advertised because operating systems and built-in apps take up space. For example, it found just 10GB was left available for the customer's songs, apps and photos on a 16GB Samsung Galaxy Tab 3.
Ben Miles of Chillbast, a UK-based computer manufacturer which buys its Flash memory from some of the same suppliers who make the Flash storage components found in tablets, told Which?: "16GB of Flash memory is mind-numbingly cheap now.
"As a general rule, for manufacturers like ourselves, Flash costs less than 40p per GB, so for companies to charge so much for an extra 16GB seems scandalous. The difference in cost between manufacturing a product with 16GB and 32GB of memory would probably equate to less than $10 (£6)."
Which? editor Richard Headland said: "With tablets in demand this Christmas, buyers will be shocked to discover what a raw deal they're getting on built-in memory. If you want the best value storage, then buy a tablet with a SD or microSD slot and add a memory card for a fraction of the cost."
Apple and Google declined to comment on the Which? report.
Samsung told Which?: "It is Samsung policy to provide the most reasonable market price to customers."