Search engines would be forced to delete personal information they collect about their users after six months under new guidelines put forward by European authorities.

A European advisory body said that it saw no justification for keeping information relating to search queries for as long as 18 months - as Google does - and criticised search engines for not being clear about the uses to which such information was put.

The Article 29 Working Party, whose recommendations are usually adopted by the European Commission, also said that IP addresses - the numbers that identify individual computers on the internet - contituted "personal information" for the purposes of data protection laws, paving the way for more close legal scrutiny of how search engines use the information they collect.

In response, Google said that it retained information about individual search queries for a number of reasons, including improving the quality of its services and preventing fraud, and suggested that it should not be forced to discard data linking such information to individual users after after only six months.

The search firm also disagreed with the working party's conclusion that an IP address constituted "personal information" for legal purposes, saying that whether such information was "personal" depended on how it was used.

The group identified a range of legitimate reasons for which search engines might retain information relating to search queries, for instance offering personal ads, assisting with law enforcement, and fraud prevention, but said that such companies had "insufficiently explained" the way they used such information to their users.

Some of the reasons put forward by search engines for retaining information about their users - for instance improving the quality of their service - were also "too broadly defined" to ascertain whether such retention was justified in any given case, the report concluded.

For instance, the report said, a search engine may want to know that user X had searched for 'Woodhouse' but then clicked the suggested variant spelling 'Wodehouse', but it did not need to know who User X was in order to adapt its service accordingly.

At present, Google, the leader in internet search, and Microsoft "anonymise" the search records of their users - meaning that they cannot be used, for example, to help deliver more personally targeted adverts to an individual user - after a period of 18 months. Yahoo!, the second largest engine, anonymises its records after 13 months.

The working party's report said: "In case search engine providers retain personal data longer than six months, they will have to demonstrate comprehensively that it is strictly necessary for the service. In all cases search engine providers must inform users about the applicable retention policies for all kinds of user data they process."

It issued a new set of obligations for search engines, including:

- search engines should require users to give specific consent before receiving more personalised adverts based on the search queries they conduct

- personal data should not be retained for longer than six months

- information stored for one purpose, such as security reasons, must not be used for other, unrelated purposes, such as optimising the search service

- in the event that a provider retains personal data for longer than six months for the purposes of security or fraud prevention, such retention should be "comprehensively justified"

In a response to the report, Peter Fleischer, Google's global privacy counsel, said: "We believe that data retention requirements have to take into account the need to provide quality products and services for users, like accurate search results, as well as system security and integrity concerns."

"We have recently discussed some of the many ways that using this data helps improve users' experience, from making our products safe, to preventing fraud, to building language models to improve search results."

In a statement, Yahoo! said that it was "committed to striking the right balance between protecting user privacy, providing the most compelling online experience, meeting our legal obligations and preventing fraud."

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