The P2B Regulation lays down the principles of fair and transparent treatment of business users using online platforms (online intermediation services). This legal act, addressing many anti-trust issues, is the first step towards creating ex ante regulatory instruments for online platforms. To contribute to the discussion on the need and possible shape of online platforms regulation, the aim of the research is: (i) to indicate the interaction between the P2B Regulation and antitrust law, with particular emphasis on the importance of antitrust law for the future ex ante regulation of large online platforms, as well as (ii) to determine the desired standard of intervention, including an effective enforcement mechanism, with regard to harmful practices of online platform operators (i.e. entrepreneurs which provide online intermediation services on the platform) identified on the basis of the P2B Regulation. Arguments for the legitimacy of public intervention in the operation of online platform operators in the case of: (i) significant bargaining power of the online platform operator over business users and (ii) damage to the public interest will be elaborated.