Google is planning to track billions of credit and debit card sales to compare online ad clicks with money spent offline.
Google Attribution will allow advertisers to see whether online ad campaigns generate offline sales.
Announcing the service, Google said that it captures around 70% of credit and debit card transactions in the US.
Critics said it represented another blow to privacy.
Google announced the new product in a blogpost, saying: “For the first time, Google Attribution makes it possible for every marketer to measure the impact of their marketing across devices and cross-channel – all in one place.”
Google has vast amounts of data on net users, from services such as AdWords, Google Analytics and DoubleClick Search which combine details about the ads displayed on devices with what has been searched for in Google.