It wasn't the Grinch who stole Christmas, it was the internet

‘Twas the night before Christmas and all over the country people were looking forward to opening their presents.

There was only one problem; they already had a fairly good idea what their parents or partner had bought them.

Why? Because when you use a tablet or family computer you are likely to notice a stream of adverts for a piece of jewellery, clothing or even cycling equipment that only you would normally be interested in - suggesting someone else has been shopping for those items

Targeted adverts are increasingly revealing secrets about what people have been searching for and buying online, potentially ruining Christmas surprises up and down the land.

The technology behind targeted, or “interest-based”, ads that pop up in banners or on the side of web pages means that when another user goes on the same device - or one that is linked through the same WiFi provider - they are likely to see adverts aimed at the previous user.

That means that if, say, your boyfriend has been browsing for engagement rings, you’ll have a pretty good idea of what’s around the corner.

And if your mother has been searching for that special outfit there’s a good chance it will be under the tree with your name on it.

Danny Sullivan, the founder of the Marketing Land blog, wrote how his wife complained that online adverts were spoiling the surprise for their children

Mr Sullivan said: “‘Everything I’m buying for Christmas, it’s showing up in ads,’ she complained to me. That’s a problem because our kids are often using her computer. You don’t want them seeing what might end up under the tree.”

Jodi Myers, of internet privacy app providers NordVPN, warns: “One family member might be browsing ‘privately’ on their smartphone, but the rest of the family might see ads on their home computer related to the other person’s mobile browsing history.

“A study shows that the majority of consumers do not realize their private data is being sold to advertising networks and third-party entities in order to provide them with targeted ads. Moreover, advertisers are using cross-device tracking, which raises additional privacy and security risks. In cross-device tracking, ad companies and publishers try to build a consumer’s profile based on their activity throughout computers, tablets, smartphones, and smartwatches.”

As one online shopper, Amanda Clayman, a financial adviser and life coach, wrote on Twitter recently: “I guess you have to pick between letting your kids use the computer and shopping online, since custom ads follow you and spoil gift surprises.”

However, consumer groups say there are a number of basic methods people can use to avoid their shopping or browsing history being seen by other people.

These include ad-blocking, which stops banner ads, pop-up ads, and other types of ads; not connecting different devices that use the same IP provider; and deleting cookies, When a user visits that website again, it will be recorded through the cookie, and targeted ads can be directed towards that person.

Users need to regularly clear their browsing data in order to get rid of all the cookies and mask their shopping or browsing habits. Online shoppers can also instruct Amazon's recommendation engine to ignore certain items they have purchased.

The consumer and privacy campaigner Big Brother Watch said: “People do need to get a bit wise about what we’re doing with their data online.

“After all, you wouldn't take your girlfriend to the pub with your mates and proceed to tell them, in front of her, how you're thinking of buying her an engagement ring. In the same way you should not shop online for a ring on a device which she is likely to use.

“If you don't want people to subsequently see what you’ve been searching for then do a private search, use an ad blocker and delete your cookies. It’s not really the internet companies or seller's’ fault you are being targeted by ads based on your browsing history. Shoppers need to take some responsibility and  get a bit smart about the way they use the internet."