Google has a "secret deal" with Mastercard that allows it to track users' shopping habits offline and link that data with adverts they see online.

Over the past year, the multi-million dollar deal has given the tech titan unprecedented insight into whether the ads it runs online translate into real-world purchases.

Users are able to opt out of being tracked. However the deal, uncovered by this week by Bloomberg, was not disclosed to the public by either Google or Mastercard.

The revelation has raised questions about how much data Google is quietly harvesting without user knowledge.

As part of the deal, when a Google user clicks on an online advert, their activity is recorded on a database, even if they don't buy that item. However, if that person uses their Mastercard to purchase the item at a physical store within one month of clicking on the ad, Google sends a report to the advertiser.

A Mastercard spokesman revealed that Google is given information that includes sales volumes and average purchase size. They denied that data could be used to identify exact purchases and said only US users are affected.

The deal with Mastercard allowed Google to launch a tool last year called "store sales measurement" designed to help companies understand how their online advertising spend has an impact on their brick and mortar sales.