Reviews and directory site Yelp has admitted that 25 per cent of its submitted reviews could be fake, with businesses increasingly trying to influence consumers' attitudes.
An automated filter suppresses around a quarter of reviews posted on the site which are deemed "suspicious" before they are made live, a spokesperson said, adding "any that are fake will be swiftly filtered out".
However the system in not perfect, the firm said, and there is a possibility that some genuine reviews may be filtered. Yelp explicitly forbids paid for reviews.
The admission follows a recently published academic study that found in 2013, at least one in five reviews posted on Yelp are fake, up from just 5 per cent in 2006.
Last week New York's Attorney General fined 19 SEO firms a total of $350,000 (£218,500) for publishing fake online reviews for other companies, an action known as 'astroturfing'.
The firms hire workers from across the world including the Philippines, Bangladesh and Eastern Europe to write the false posts for $1 (62p) to $10 (£6.20) a time.
"The problem is definitely more widespread than the Attorney General's investigation," Harvard University's Michael Luca, who co-authored the report, told Market Watch.
Luca and his team analysed over 310,000 reviews of 3,625 restaurants. They found that both fake positive and fake negative reviews were being posted. Whilst the positive ones were to boost reputation and counteract bad reviews, the negative ones were designed to work against competition.
"As crowdsourced information becomes increasingly prevalent, so do incentives for businesses to game the system," the report said.