This could involve, say, employing people in India to leave positive reviews for hotels they have never stayed at!

Such Astroturf is easy to spot for the discerning traveller. It's not difficult to see that Trevor, from Maidstone is genuine as his five star rating for the hotel's full English, judging by the girth displayed on his profile picture. It also obvious that reviews full of uninformative platitudes with dodgy Eastern European English, are fake.

But when done well, by less scrupulous online PR companies, the fake reviews can go unnoticed. The technique is to build dozens of profiles with fake background histories then, using 'persona management' software, drown out real opinion on review and forum websites.

The problem is that even obviously bogus positive reviews all add up to promote hotels to the top of TripAdviser search results. More worryingly, negative ratings can suppress a competitor's listings to low ranking obscurity.

Given the increasing sophistication of its perpetrators, spotting Astroturf is very hard to automate. The most effective way a website can remove it is to ask users to flag any reviews that they think are synthetic.

A real review will have good and bad elements; often the negative comments will lend validation to the positives.

Unfortunately, Astroturfing is not limited to hotel review sites. Consumers now seek online opinion for all major purchases. Thanks to social media, we now trust the collective opinion of the online crowd more than any other.

This trust may become misplaced if websites like TripAdviser don't find some way to combat the rise of the Astroturfer.

Chris MD of Social Media and online PR agency Friend Digital www.frienddigital.com , chris@frienddigital.com, @ChrisTomlinson1