BRUSSELS (Reuters) - Amazon ( AMZN ) will likely face an EU investigation next year into whether it favours its own brand products on its online marketplace as European antitrust regulators build up a case under landmark rules, people with direct knowledge of the matter said.

The U.S. online retailer could face a fine of up to 10% of its global annual turnover if it is found guilty of breaching the European Union's Digital Markets Act (DMA) which seeks to rein in the power of Big Tech.

The timing and decision on whether to go ahead with an investigation will be taken by incoming EU antitrust chief Terera Ribera in the coming months, the people said. The Spaniard is set to take up her post next month, replacing outgoing Margrethe Vestager.

Amazon ( AMZN ) said it is compliant with the DMA and has engaged constructively with the Commission on its plans since the designation of two of its services as important gateways between businesses and consumers and thus subject to the new rules.

The DMA, which came into effect last year, prohibits Amazon ( AMZN ) and six other Big Tech companies from giving preference to their products and services on their platforms, among other requirements.

The European Commission, which acts as the EU antitrust enforcer, in March said it was gathering facts and information on Amazon's ( AMZN ) treatment of its own brand products on the Amazon Store.

It declined to comment on Thursday.

In its compliance report published in March, Amazon ( AMZN ) said its ranking models do not differentiate on whether the product is sold by Amazon Retail or a Seller or whether it is an Amazon ( AMZN ) product or a third-party product.

Apple, Alphabet's Google and Meta Platforms are currently being investigated under the DMA, with Ribera set to decide on the outcomes and not Vestager, one of the people said.