MILAN—Google Inc., Apple Inc. and Amazon.com Inc. are being investigated by an Italian regulator to determine whether the Internet giants mislead people to make purchases in mobile phone apps that are initially free to download.

The move by Italy’s Antitrust and Competition Authority comes after the European Union earlier this year called on companies to reform their use of the “freemium” model in which apps are free to download, but then later require payments that often get charged to credit cards by default. The EU says consumer confusion with the freemium model threatens the long-term health of the continent’s booming “app economy” that employs more than one million people and is forecast to produce €63 billion in total revenue in 2018, more than triple last year’s level.

The Italian regulator said Friday it is investigating whether the three U.S. Internet companies—which run stores to download apps—and Gameloft, a French game developer, are misleading consumers with their use of the freemium model, which accounts for more than half of the EU online games market.

Purchases in freemium apps can include access to new levels in a game or virtual objects, such as a sword, to make the player more powerful.

“Consumers could be led to think, contrary to reality, that a game is completely free and therefore they don’t know ahead of time the game’s true cost,”