LONDON — If data is the most valuable currency of the digital economy, at what point does a company have so much that it becomes unfair?

That’s a question antitrust experts are increasingly asking themselves as the world’s biggest technology companies harvest more and more information about people and businesses. On Monday, European regulators pushed the idea forward, announcing an investigation into Apple’s proposed acquisition of the song-identification app Shazam over concerns the iPhone maker would get access to data on competitors like Spotify.

Antitrust cases, particularly in the United States, are typically argued over the impact a deal will have on consumers, such as the price of a product or service. Margrethe Vestager, the European Union’s top antitrust official, has argued data should become more of a factor.

Ms. Vestager has said that with free services, customers pay with their data, and are not always getting a fair deal. This broader interpretation of antitrust law would have important consequences for future acquisitions made by companies such as Amazon, Facebook and Google.