Leading article: No escape from Sir Martin's watchful eyes

When you make your next purchase online, Sir Martin Sorrell will probably get to learn of it. The Briton who heads the world's biggest advertising group, WPP, yesterday launched a new company, Xaxis, which will preside over the "world's largest database" of 500 million personal profiles, based on such things as the individual's choice in websites and purchases made online. Xaxis will track "almost 100 per cent of the UK population".

It is potentially a breakthrough moment in advertising. The service promises to enable businesses to target potential customers for products and services with greater precision than ever before. Advertisers will be able to address specific individuals across all digital platforms rather than via a single medium known to appeal to a certain demographic.

There may be great economic benefits to this approach. Rather than wasting marketing budgets on misdirected campaigns, companies can be sure of putting their goods before potential customers who have indicated some interest in purchasing them. For the consumer there is something attractive in being alerted to offers of products and services that accurately match our tastes. Few people object to the buying "suggestions" now given by many online businesses, based on our previous purchases.

But there is something deeply unsettling about this development. It is one thing to follow a user's purchasing patterns within a single trusted site, something else to compile an extensive record of an individual's shopping habits. One American headline warned that Sir Martin's company "has your profile".

Xaxis, we are assured, keeps a database which is anonymous and fully secure. It is the audience that clients are after, not the individual. But hackers have become increasingly sophisticated, as has been demonstrated by a series of recent data breaches where victims have included such corporations as Sony and Lockheed. Privacy campaigners warn that the risks are posed not by the compilers of the data but by those who might subsequently obtain it illicitly. This development should be watched closely, not least by the Information Commissioner.