Following years of negotiations, Google paid millions of dollars for the Mastercard data, according to Bloomberg, citing numerous sources involved in the deal.

Since last year, select retailers have had access to a tool called Store Sales Measurement, which lets them track the link between online ads and in-store sales.

The arrangement, which customers were not made aware of, gives Google a valuable insight into how merchant advertising is linked to customer spending that is not online.

Google and Mastercard declined to comment on their specific partnership.

However, a Google statement to Bloomberg on the advertising tool says: "Before we launched this beta product last year, we built a new, double-blind encryption technology that prevents both Google and our partners from viewing our respective users’ personally identifiable information.

"We do not have access to any personal information from our partners’ credit and debit cards, nor do we share any personal information with our partners."

Mastercard says that no individual transaction or personal data is handed over, adding that the way that it's network operates means that it does not know the the individual items that a consumer purchases in any shopping cart – physical or digital.

Instead, a "small group" of test advertisers see aggregate sales figures and estimates of how many are down to Google ads. The data only applies to people who are logged into a Google account and have not opted out of ad tracking.

Sources told Bloomberg that Google talked to other payment companies about similar deals, but it is not known whether any agreements were reached.