​The European Union fined Meta roughly $840 million on Thursday for breaking competition laws with Facebook Marketplacevvjl, its shopping and classified ads platform, the latest action by regulators trying to limit the ability of tech giants to expand into new product areas.

In issuing the 800-million-euro fine, European regulators said Meta had given itself an unfair advantage over rival services by bundling Marketplace into Facebook’s wider social network, providing it with immediate access to millions of potential users. They added that Meta had abused its dominance in online advertising to impose unfair business terms on rival shopping services, allowing it to collect data that could be used to strengthen Marketplace.

European regulators, led by Margrethe Vestager, the E.U. competition chief, have for years sought to limit the ability of tech companies to use their power in one area, like social networking, to gain a foothold in new markets such as shopping. Authorities in Europe have also accused Apple of using its dominance in smartphones to bolster music and payment services.

The fine against Meta is one of the final acts by Ms. Vestager before the end of her term at the European Commission, the executive branch of the 27-nation bloc. After a decade steering competition policy and pushing for tougher regulation of the tech industry, she is slated to be replaced in the coming weeks by Teresa Ribera, a Spanish official.

In linking Marketplace to Facebook’s social network, the company gave itself “advantages that other online classified ads service providers could not match,” Ms. Vestager said in a statement. “This is illegal under E.U. antitrust rules. Meta must now stop this behavior.”

The company said it would appeal the decision, setting up a legal battle that could drag out for years. Meta said Marketplace, introduced in 2016, was created in response to consumer demand and had not hindered competition from companies such as eBay and Vinted.

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