The inquiry, which by its nature will inevitably focus heavily on U.S. firms, follows calls from France and Germany for regulation of so-called "essential digital platforms", encompassing everything from e-commerce sites such as eBay to social media companies.
European Commission Vice-President Andrus Ansip will unveil his "Digital Single Market Strategy" at noon (1000 GMT) in Brussels. It will aim to update copyright rules, knock down barriers to cross-border parcel deliveries and ensure European online businesses can compete with their bigger U.S. counterparts.
The inquiry differs from an antitrust investigation of the kind launched by the EU into Google five years ago, in that it is not aimed at enforcing existing law through penalties.
Rather it will look at whether Internet platforms are transparent enough in how they display search results and if they promote their own services to the detriment of competitors, according to a draft of the strategy seen by Reuters.
Politicians and businesses across Europe have been calling for the market power of dominant U.S. tech firms to be curbed to help Europe's fledgling web industry compete, leading to accusations of protectionism from U.S. President Barack Obama.