The latest Snowden leak suggests US and UK cyberspies are taking advantage of Google’s proprietary cookie technology in an effort to track suspects. http://www.bbc.co.uk/news/technology-25333663   Documents published by the Washington Post refer to the NSA and GCHQ’s use of “GooglePrefIDs” – files containing a numeric code placed on computers to help the search firm remember users.

The paper said the US and UK spy agencies piggybacked the files to “home in” on targets already under suspicion.

Google has not commented.

However, the news may add to existing tensions between the firm and the authorities.

Google’s chairman Eric Schmidt said last week that the company had considered moving its servers outside of the US following the publication of earlier leaks, before deciding it was impractical.

The cookie surveillance technique is the latest in a series of alleged spy agency activities described by papers released to journalists by the whistleblower Edward Snowden, a former NSA contractor now living in Russia.

Google says it uses “preferences” cookies to enhance people’s use of the internet.

“These cookies allow our websites to remember information that changes the way the site behaves or looks, such as your preferred language or the region you are in,” it explains on its site.

The file – which contains a randomly-generated numeric code, rather than the name of the user – is also used by the firm to personalise the adverts shown to people who are not signed into its service.

Since many other firms make use of Google’s technologies to place ads, a user may have PrefIDs on their computer even if they have never visited the search firm’s own services.

There are tools on the internet with which users can reset the cookie’s numeric code to make themselves anonymous. One expert said the company would be concerned if the leaks encouraged more people to use them.

“The last thing that Google wants is for people to tamper with or otherwise mess with its tools, disabling its ability to track them,” said Chris Green, a tech analyst at the consultancy Davies Murphy Group.

“Cookies are a very valuable part of its business.”

A document published by the Washington Post suggests the spy agencies also track other types of cookies, but does not specify which.

It is not clear how the authorities would have obtained the information, although the paper notes that it is among the data the NSA can demand through a Foreign Intelligence Surveillance Act (Fisa) court order.

A spokesman for GCHQ said he could neither confirm nor deny the agency’s involvement in the alleged activity.