PARIS — What does Google know about its users and how does it know it? European privacy regulators on Tuesday warned the company to clarify those issues — or risk fines or other penalties by early next year.

In a letter to Larry Page, the chief executive of Google, 27 European data-protection agencies asked the company to modify its global privacy policy that governs dozens of Google online services — including the flagship search engine, Android mobile phone apps and YouTube videos — so that users have a clearer understanding of what personal data is being collected and can better control how that information is shared with advertisers.

Along with other Internet companies like Facebook and Microsoft, Google collects personal data, like the sex and age of users and their Web browsing histories, in order to tailor their services to individual users and also to sell ads.

When Google introduced the privacy policy last winter, it described it as a way to streamline its use of personal data across a range of services that were each previously covered by separate privacy guidelines. And in keeping with European privacy law, Google said it was collecting the data only if users “opted in.” But opting in essentially became a requirement of using each of the services, by clicking the “I Agree” button before using the service for the first time, after the new policy went into effect.