Google plans to track credit card spending so it knows when you are buying things in shops - as well as online

File photo: A Google search page is reflected in sunglasses in this photo illustration taken in Brussels May 30, 2014. (REUTERS/Francois Lenoir)

Google has revealed “intrusive” new plans to track what we are buying in shops.

It will use credit and debit card information to prove that its online adverts can get shoppers to the till.

Determining how many sales are generated by digital advert campaigns has long been the “holy grail” among industry insiders.

Google has previously used web browsing, search history and geographic locations to see if an ad has been successful.

But adding transactions in shops to the mix could help Google show clients that buying advertising is a good idea.

Virgin Holidays said it has already benefited from tracking sales both on and offline.

It found that those who buy in store after clicking on a search advert are “three times more profitable than an online conversation”, which means that people who buy instore – but previously saw or clicked on a Google advert – spend more than people who just click through an advert to buy online.

It’s easy to forget that Google is primarily a massive advertising company that makes billions from our eyeballs.

But the new powers to track our offline spending has caused a stir with privacy advocates.

They claim that customers haven’t got the faintest idea that their transactions are being tracked, and more must be done to alert them.

“What’s really fascinating to me is that as the companies become increasingly intrusive in terms of their data collection, they also become more secretive,” Marc Rotenberg, executive director of the Electronic Privacy Information Center told the Washington Post.

The search giant said it is in partnership with companies who track 70 percent of all credit and debit card transactions in the United States.

It can merge this information with the data it owns which shows who has seen or clicked on an advert in that location.

Google said it puts user privacy first.

"While we developed the concept for this product years ago, it required years of effort to develop a solution that could meet our stringent user privacy requirements," it said in a statement.

"To accomplish this, we developed a new, custom encryption technology that ensures users' data remains private, secure, and anonymous."

The UK government gave Google a slap on the wrist after it emerged adverts - footed by the British taxpayer - were appearing on terrorist propaganda YouTube clips.