Google is bracing itself for a fresh regulatory review after privacy campaign group Epic lodged a complaint with America’s Federal Trade Commission in a bid to stop the tech giant’s Store Sales Measurement service.

Created to track what customers spend offline to support its advertising sales with real-world data the practice has outraged privacy campaigners who fear it is an infringement too far on personal space.

Launched in May the advertiser tool enables marketers ‘to measure in-store revenue in addition to the store visits delivered by your Search and Shopping ads’ by linking around 70% of all US credit and debit card transactions with specific online campaigns.

Armed with such proof of purchase Google can command a higher premium when negotiating advertising sales but Epic contends that this comes with an unacceptable loss of privacy in relation to consumers purchases, health and private lives.

In its legal filing Epic stated: “The privacy of millions of consumers thus depends on a secret, proprietary algorithm and although Google claims that consumers can opt out of being tracked, the process is burdensome, opaque, and misleading."

Google insists that it employs anonymizing software to prevent such outcomes, telling The Register: “All data is encrypted and aggregated – we don’t share or receive any identifiable credit card data whatsoever. Users have robust controls – we only use data that they’ve consented to have associated with their Web and App activity in their Google account, which users can opt-out of at any time.”

The FTC have yet to decide whether any enforcement action is justified in this case, although the firm hasn't escaped unscathed from past brushes with the law having been landed with a €2.4bn antitrust fine in Europe.