Chinese regulators have fined giant e-commerce giant Alibaba 18.3 billion yuan ($2.8 billion) on charges of violating anti-monopoly rules.

The ruling Communist Party is tightening control over China’s biggest e-commerce and other internet companies and has warned them not to use their industry dominance to stifle competition.

The State Administration for Market Regulation announced Saturday that Alibaba was fined for “abusing its dominant position” to limit competition in online retailing.

Alibaba is the world’s biggest e-commerce company by total volume of goods sold across its platforms.

The move is a new setback for Alibaba and its billionaire founder, Jack Ma, following a November decision by regulators to suspend the stock market debut of Ant Group, a finance platform spun off from the e-commerce giant. It would have been the world’s biggest initial public stock offering last year.

Ma, one of China’s richest and most prominent entrepreneurs, disappeared temporarily from public view after criticizing regulators in a November speech. That was followed days later by the Ant Group suspension, though finance specialists said regulators already had been worried Ant lacked adequate financial risk controls.

Alibaba, launched in 1999, operates retail, business-to-business and consumer-to-consumer platforms. It has expanded at a breakneck pace into financial services, film production and other fields.

Twelve companies including Tencent Holdings, which operates games and the popular WeChat messaging service, were fined 500,000 ($77,000) each in March on charges of failing to disclose previous acquisitions and other deals.

In February, the government issued anti-monopoly guidelines aimed at preventing anti-competitive practices such as signing exclusive agreements with merchants and use of subsidies to squeeze out competitors.

Regulators said in December they were looking into possibly anti-competitive tactics by Alibaba including a policy dubbed “choose one of two,” which requires business partners to avoid dealing with its competitors.