The EU Commission on Thursday fined US tech giant Meta €797.7 million ($840.2 million) for breaches of EU competition law.
The body said that Meta had engaged in practices benefiting Facebook Marketplace.
Besides the Facebook social media platform, Meta also owns Instagram and the WhatsApp messaging service.
"The European Commission has fined Meta ... for breaching EU antitrust rules by tying its online classified ads service Facebook Marketplace to its personal social network Facebook and by imposing unfair trading conditions on other online classified ads service providers," the European Commission said.
In response, Meta said the EU offered no evidence of harm to competitors or consumers and vowed to appeal the decision.
"Facebook users can choose whether or not to engage with Marketplace, and many don't. The reality is that people use Facebook Marketplace because they want to, not because they have to," the firm argued.
The firm said that it would comply with the decision in the meantime and would work quickly and constructively to launch a solution that addresses the points raised.
The penalty comes two years after the Commission accused Meta of giving its classified ads service Facebook Marketplace an unfair advantage by tying it to the Facebook social network.
Meta launched Facebook Marketplace in 2016 and expanded into several European countries a year later.
This is a developing news story and will be updated.
sdi/ab (AP, AFP, dpa, Reuters)