Google and Mastercard have reportedly struck a secret deal to allow merchants to track when online ads have led to sales at bricks and mortar stores.

Google’s parent company, Alphabet Inc and Mastercard brokered the partnership during protracted negotiations lasting about four years, according to sources cited by Bloomberg.

The sources, two of whom claimed to have worked directly on the project, said that the tie-up - for which Google paid millions of dollars - allowed the search engine to use Mastercard data to create an unprecedented method for analysing retail spending linked to adverts displayed on its platform.

The tool, called Store Sales Measurement, was in part linked to a stockpile of Mastercard transaction data bought by Google. According to Bloomberg, customers were not made aware of the arrangement which has reportedly been in place since last year.

A Google spokesperson said: “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.

They added: “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. Google users can opt-out with their Web and App Activity controls, at any time”.

A spokeswoman for Google also told Bloomberg there was no ad-based revenue sharing agreement between the partners.

A Mastercard spokesman also declined to comment specifically on a partnership with Google, but said in a statement released to Bloomberg that that no personal data or individual transaction details are handed over.

"We do not provide insights that track, serve up ads to, or even measure ad effectiveness relating to, individual consumers," he is quoted as saying.

Share Story: