Amazon has been fined $1.2bn (£910m) by Italy’s anti-trust regulator.
It said the tech giant had abused its market dominance by promoting its own logistics service, Fulfilment by Amazon (FBA).
It claimed companies had to use the FBA service to access key benefits such as selling products with Prime delivery with no extra cost to customers, and participation in Black Friday sales.
Amazon said it “strongly disagreed” with the decision, and would appeal.
The regulator ruled that Amazon put third-party sellers at a disadvantage by requiring use of its own service to access key benefits and events.
“Amazon has thus prevented third-party sellers from associating the Prime label with offers not managed with FBA,” it said.
The regulator insisted that access to such functions is “crucial” for sellers to achieve success on Amazon’s Marketplace.
It also said it would impose corrective steps which will be subject to review by a monitoring trustee.
Amazon said in a statement that the fine was “unjustified and disproportionate”.
“We strongly disagree with the decision of the Italian Competition Authority and we will appeal,” said the company.
“Small and medium-sized businesses have multiple channels to sell their products both online and offline: Amazon is just one of those options.
“We constantly invest to support the growth of the 18,000 Italian SMBs [small and medium-sized businesses] that sell on Amazon, and we provide multiple tools to our sellers, including those who manage shipments themselves.”
The company added that third-party sellers can use its Seller Fulfilled Prime (SFP) service, which gives them access to Prime benefits without having to use its own logistics services.
It marks the second fine against Amazon by Italian regulators in a matter of weeks after both it and Apple were fined $228m (£173m) for restricting Beats headphone sales, by limiting them to select retailers.
Both Apple and Amazon said they plan to appeal against the fines.