The Amazon website is seen on December 5, 2017 in Dandenong, Australia.

Amazon is flooded with thousands of fake five-star reviews, an investigation published Tuesday has claimed.

The probe, carried out by U.K. consumer advocacy group Which?, analyzed hundreds of tech products on the site and found that potentially false reviews were helping unknown brands dominate searches for popular items.

According to Which?, sellers were listing products that carried tens of thousands of positive unverified reviews – meaning there was no evidence the people leaving the reviews had bought the product on Amazon or elsewhere.

Hundreds of unverified five-star reviews were being posted on product pages in a single day, the investigation found. Many product pages also included positive reviews for completely different items.

Which? searched Amazon for 14 tech products, including headphones, smart watches and wearable devices. The first page of search results for headphones, when sorted to display the products with the best reviews first, showed that 100% of the items were being sold by brands that Which?'s tech experts had never heard of.

While 71% of the items on the first page of search results had five-star reviews, almost 90% of those reviews were unverified.

In just a couple of hours, Which? uncovered more than 10,000 reviews from unverified purchasers on just 24 items. The organization described this as "an easy-to-find red flag that highlights the scale of Amazon's problem with fake reviews."

One pair of headphones being sold by an unknown brand had 439 reviews – all of which were five-star, unverified, and posted on the same day.

Independent site ReviewMeta – which examines reviews on Amazon – said in the report that it believed every unverified five-star review on the top ten pairs of headphones was fake.

"I'm shocked we've been seeing this so much on Amazon – (it) seems so obvious and easy to prevent," a spokesperson said.

Other products, such as fitness trackers and smart watches, showed the same "suspicious activity."

An Amazon spokesperson said in a statement that the company invested "significant resources" to protect the integrity of reviews on its platform.

"Even one inauthentic review is one too many. We have clear participation guidelines for both reviewers and selling partners and we suspend, ban, and take legal action on those who violate our policies," they said.

As many as 97% of shoppers rely on online reviews to help make a purchase, according to research conducted by Which? in September last year. Britain's Competition and Markets Authority estimates that £23 billion ($30 billion) of the U.K.'s consumer spending is influenced by online reviews every year.

According to a study on U.S. consumer behavior by Northwestern University's Spiegel Research Center, online reviews have the power to increase purchase rates by as much as 380%.

Which? is an independent consumer body that also conducts its own reviews into household products.