A warning for armchair epidemiologists: people who make unsubstantiated allegations about food poisoning in reviews on user-generated websites such as TripAdvisor could face legal action.

“It’s almost impossible to say with any certainty that food poisoning came from any one meal, so making these kind of threats could potentially be libellous,” said Mark Harrington, chief executive of Check Safety First, a company specialising in food hygiene checks.

Mr Harrington told The Telegraph that fake restaurant reviews are being used to blackmail hoteliers. “There have been many reports that customers have blackmailed hoteliers by threatening to post false food-poisoning claims on TripAdvisor. It is scandalous.”

The news follows the Advertising Standards Authority’s (ASA) ruling that TripAdvisor can no longer claim or imply that all its reviews can be trusted.

Kwikchex, a reputation management company that brought the case to the ASA on behalf of hoteliers and restaurateurs, said there were thousands of such allegations of food poisoning in Britain and U.S.

“Almost none are reported to the proper authorities, let alone substantiated,” said a spokesman. “Sometimes the reviewer believes it is the truth, but has not reported it and has no understanding of gastro-intestinal infections.

“They usually just pick on the last place where they ate, when in fact the incubation period for such infections is usually one to two days and sometimes as long as a week.”

The spokesman added that this type of allegation can be used by competitors and disgruntled ex-employees to harm the business.