US search giant Google is quietly keeping track of every single purchase you have made in years, thanks to purchase receipts sent to your personal Gmail account.

According to a report in CNBC on Friday, the information is made available to users via a private web tool which, the company claims, keeps your data private.

Google, however, stressed it does not use the information for personalised ad tracking. In 2017, the tech giant said it would stop using data collected from Gmail messages to personalize ads.

"To help you easily view and keep track of your purchases, bookings and subscriptions in one place, we've created a private destination that can only be seen by you," Google told The Verge in a statement.

"You can delete this information at any time. We don't use any information from your Gmail messages to serve you ads, and that includes the email receipts and confirmations shown on the Purchase page," the company added.

Google, however, did not say how long this web tool has been active. The company on May 14 announced it will now allow ads to reflect on the homepage of its mobile app.

The ads would be displayed on the "Discover" section of the homepage in a news feed layout and would be algorithmically personalised. The search-engine giant also plans to then place ads with up to eight pictures in its search results as part of its "Gallery ads" feature.

Ads would also appear on the homepage of Google Shopping, which users would be able to personalise and filter based on features they care about and brands they love.