As many as 700 owners of guesthouses, B&Bs and hotels are joining forces over what they see as unfair reports.

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Central to any case will be whether TripAdvisor, which is based in the city of Newton, Massachusetts, and is part of the online travel company Expedia, can be held liable because its business is based on publishing user-generated content - the opinions of others. The communications decency act of 1996 in the US has been taken to mean that operators of websites are not legally liable for the words of third parties.

Chris Emmins, co-founder of KwikChex, said it would be presenting TripAdvisor with "the most severe cases" and give them 14 days to take action. If they did not, lawyers would begin legal proceedings against TripAdvisor - and would request the release of information about people posting "defamatory" reviews.

Emmins said the protection offered to both hosts of reviews and to anonymous posters was "crumbling" with some courts in America insisting that "due care" needs to be taken by both.

TripAdvisor refuses to comment about any of the cases that are the subject of "threatened or pending litigation".

But a spokeswoman said: "We believe our more than 35m reviews and opinions are authentic and honest from real travellers, which is why we enjoy tremendous user loyalty and growth. If the reviews people read didn't paint an accurate picture users would not keep coming back."

Every review is screened by online tools and dozens of "quality assurance specialists" investigate "suspicious reviews". Hoteliers are given the chance to post a management response to reviews. It advises travellers to throw out the "anomalies that appear overly critical or overly complimentary" when making a choice.