Google has reportedly bought Mastercard credit card data in the US to help it track users’ offline spending in stores.
The two firms had not made the deal public but it was discovered by Bloomberg.
Mastercard denied suggestions that its data could be used to identify exact purchases.
The Open Rights Group told the BBC the confidential nature of the deal raised privacy issues.
“This raises serious concerns regarding the use of private financial data,” said legal director Myles Jackman.
“Will Mastercard be compensating their clients for the data they have given away to Google for their own financial gain?”
Google says all the data is anonymised and that users can opt out of ad tracking by switching off the web and app activity control.
It is testing a service for ad buyers in the US that shows how digital ads influence in-store spending.
On its website, the firm claims that advertisers who qualify to use its “store sales management” service can see whether an ad click or video view results in an in-store purchase within 30 days.
Google said the service was a test product in the US and only available to certain ad buyers.
It launched the ad tool in 2017.