The lawsuit will assert Amazon abuses its dominant position and harms consumers by directing them to their "featured offer" in a so-called Buy Box, with products selected via a "secret algorithm."
The class action suit against the global marketer will be filed by consumer rights advocate Julie Hunter by the end of October at London's Competition Appeal Tribunal, according to the statement from Hunter and law firm Hausfeld.
"Many consumers believe that Amazon offers good choice and value," Hunter's statement said, "but instead, it uses tricks of design to manipulate consumer choice."
Hunter added "Far from being a recommendation based on price or quality, the Buy Box favors 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."
Hunter added that the US giant "shouldn't be allowed to set the rules in its favor and treat consumers unfairly."
Amazon's response to the impending suit was ""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," the company insisted.