The European Union's data watchdog will expand its investigation of Web search engines beyond sector leader Google and write to that company, a European Commission source said on Thursday.

The Article 29 Data Protection Working Party will prepare a substantial letter of response to the letter of Google and they have also decided they will look into other search engines, the source said.

The source was referring to a letter Google wrote last week saying the company was ready to curtail the time it stored user data to a year and a half. It was seeking to mollify the watchdog, which had questioned its privacy policies last month.

That storage time was the low end of an 18- to 24-month period it had originally proposed to regulators in March.

A draft response by the working party is likely to be considered when it meets again in September, the source said.

The watchdog, made up of national data protection supervisors of the EU's 27 countries, said in May that Google seemed to be failing to respect EU privacy rules and asked for clarification before its meeting on Wednesday this week.

The body advises the EU on data protection issues, but its opinions are not legally binding.