Amazon shoppers could be in line for £900million in compensation, as lawyers claim the retail giant encouraged customers to overpay.
Legal experts say tens of millions of consumers could be affected - but Amazon strongly denies the allegation.
Law firm Hausfeld has launched a legal challenge against Amazon, which it claims has breached competition rules.
Hausfeld says Amazon has directed shoppers on its website and app towards products that were better for Amazon - but sometimes hid cheaper items.
It claims these products are either sold by Amazon itself, or by sellers who pay Amazon a fee to use its delivery service.
"By promoting the offers more favourable to Amazon and excluding other sellers, who may be offering the same product cheaper or on better terms, customers are enticed to pay more than they need to and more restricted in choice," Hausfeld said.
Specifically, the law firm says Amazon used its "Buy Box" section to promote deals that made it more money.
"Potential purchasers are steered towards the 'Featured Offer' in the Buy Box which is the only offer considered and selected by the vast majority of users, many of whom trust Amazon and wrongly assume it is the best deal," Hausfeld explained.
This alleged practice prevents shoppers from easily finding cheaper deals or delivery, the law firm said.
"Other would-be sellers are nearly always excluded from the Buy Box, stifling their ability to offer consumers a better deal, and leaving consumers out of pocket," the law firm went on.
The claim will accuse Amazon of breaching section 18 of the UK Competition Act 1998 and Article 102 of the Treaty on the Functioning of the European Union.
Hausfeld said anyone who lives in the UK and bought something from Amazon since October 2016 could be eligible.
The law firm wants to bundle Amazon customers together for the claim - but consumers don't need to do anything to opt in, as the claim is being made on their behalf.
Shoppers can keep track of the progress of the claim on this website.
Independent consultant and consumer champion Julie Hunter is representing Amazon shoppers in the claim.
Hunter said: "Many consumers believe that Amazon offers good choice and value, but instead it uses tricks of design to manipulate consumer choice and direct customers towards the featured offer in its Buy Box.
"Far from being a recommendation based on price or quality, the Buy Box favours products sold by Amazon itself, or by retailers who pay Amazon for handling their logistics.
"Other sellers, however good their offers might be, are effectively shut out – relegated down-page, or hidden several clicks away in an obscure corner of Amazon’s website."
Lesley Hannah, a partner at Hausfeld, said: "Most consumers use the Buy Box when purchasing products on Amazon – estimates range from 82% to 90%.
"This means that millions of consumers have paid too much and been denied choice. This action seeks fair redress for them."
An Amazon spokesperson said: "This claim is without merit and we’re confident that will become clear through the legal process.
"Amazon has always focused on supporting the 85,000 businesses that sell their products on our UK store, and more than half of all physical product sales on our UK store are from independent selling partners.
"We always work to feature offers that provide customers with low prices and fast delivery.”
