European regulators have become more critical of Google's business practices in the past year, including French and Spanish authorities who say Google's policies do not comply with their data protection rules.
Privacy advocates say that questions about how US firms protect European data will only become more pointed in the wake of revelations about a National Security Agency surveillance program, PRISM, that targets foreign data on the servers of US companies.
The surveillance program "combined with the ongoing investigation of Google's business practices, has created almost a tsunami of privacy enforcement," said Marc Rotenberg, executive director of the advocacy group Electronic Privacy Information Centre.
The information agency said that it was particularly concerned that Google's policy, which went into effect in March and covers more than 60 Google services, does not give users enough information about the data the firm collects and how it is used. It is also concerned that the policy does not disclose enough information about how long Google keeps user data.
If Google does not amend its policy, the ICO said it will "leave the company open to the possibility of formal enforcement action."
Last month, after a year-long investigation led by French data protection authority CNIL, French and Spanish regulators said Google could face fines if it did not amend its privacy policies within three months. The agency also indicated data protection officials in the Netherlands and Italy were assessing whether Google violates data protection rules in those countries.
Officials from the European Commission have warned that the fallout from the PRISM program could discourage European businesses from using American data storage providers such as Google, Microsoft and Dropbox.
On Wednesday, members of a key European Parliament committee voted to launch an inquiry into U.S. data surveillance programs and said they were open to suspending data-sharing deals with the United States.
Data privacy is also expected to be a part of US-EU trade negotiations that are scheduled to begin Monday.
Read more: Google hits more privacy flak in Europe