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, 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 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.