The consumer group says it found 10 sites offering the service that can be bought by third-party sellers who use Amazon’s services.

Even though the practice is firmly against Amazon’s terms and conditions, Which? said the fake review industry was thriving.

All of the 10 sites offering review manipulation services that Which? investigated were found easily on the first page of Google search results for terms like ‘make money from reviews’ or ‘get free products’.

Some were even sponsored adverts, meaning the business has paid Google to appear at the top of the results and that the ad content has been reviewed by the search engine.

Which?’s investigation points to the sheer scale of the fake reviews industry – with 702,000 product reviewers across just five of the businesses it looked at.

For sellers looking to buy reviews there was an array of different packages they could purchase to boost their products on Amazon – and Which? saw several examples of products where these tactics appeared to have paid off.

Product reviewers who sign up get access to hundreds of products, from children’s toys to exercise equipment, and are offered small payments alongside a free or discounted product – ranging from a few pounds up to more than £10. They even have the option of taking part in loyalty schemes and earning themselves the pick of premium products.

One group, German-based AMZTigers, sells reviews to sellers individually for around £13 or in bulk packages, starting at £620 for 50 reviews and going up to an eye-watering £8,000 for 1,000 reviews. It claims to have a large army of reviewers – 62,000 globally and 20,000 UK-based.

Natalie Hitchins, head of Home Products and Services at Which?, said: ‘More people are shopping online than ever before due to the coronavirus crisis – yet our latest research shows that Amazon is facing an uphill struggle against a relentless and widespread fake reviews industry geared towards misleading consumers.

‘The regulator must crack down on bad actors and hold sites to account if they fail to keep their users safe. If it is unable to do so, the government must urgently strengthen online consumer protections.

‘Amazon, and other online platforms, must do more to proactively prevent fake reviews infiltrating their sites so that consumers can trust the integrity of their reviews.’

Amazon said it was continuing to try and crack down on fake reviews

An Amazon spokesperson said: ‘We remove fake reviews and take action against anyone involved in abuse. We have won dozens of injunctions against providers of fake reviews across Europe and we won’t shy away from taking legal action.

‘However, Amazon and other online retailers cannot do this alone. Customers need to be able to trust the reviews they see online and the systematic manipulation of reviews needs consistent enforcement and global coordination with stronger enforcement powers given to regulators against bad actors.

‘We continue to work to protect the authenticity of customer reviews. We advise customers who doubt the credibility of a review on a product to click the ‘report abuse’ link available below each review.. We will then investigate and take necessary measures.’

Amazon Marketplace is the world’s biggest online marketplace and a lucrative prospect for third-party sellers. It sold an estimated £215 billion worth of products worldwide in 2020.