Yelp, the online directory and consumer review site, says a quarter of the reviews it receives could be fake, as businesses increasingly attempt to skew consumers' opinions.
Its automated review filter now "suppresses around 25%" of "suspicious" reviews it receives, the company said.
"Any that are fake will be swiftly filtered out," said a spokesman.
But the company admitted its system was not foolproof, and that some filtered reviews could be genuine.
Last week New York authorities fined 19 firms a total of $350,000 (£218,500) for posting fake reviews on websites.
In a year-long sting operation, the city's attorney-general's office set up a fake yogurt shop in Brooklyn and asked for help from firms offering search engine optimisation (SEO) services that can help boost companies' online presence.
Some of these firms created online profiles and then paid for reviews from freelance writers, some from the Philippines, Bangladesh and Eastern Europe.
Many businesses were also producing their own fake online reviews, the authorities discovered.
The practice is known as "astroturfing", reflecting the fact that such consumer - or grass-root - reviews are not genuine.
A recent academic report found that the proportion of fake reviews submitted to Yelp had risen from 5% in 2006 to 20% in 2013.
Michael Luca of Harvard Business School and Georgios Zervas of Boston University studied the incidence of fraudulent reviews of Boston restaurants posted to Yelp, including those that had been filtered out.
After analysing more than 310,000 reviews of 3,625 restaurants, they found that negative fake reviews occurred in response to increased competition, while positive fake reviews were used to strengthen a weak reputation or to counteract unflattering reviews.
Fake reviews tend to be extremely positive or negative, they found.
"As crowdsourced information becomes increasingly prevalent, so do incentives for businesses to game the system," the authors concluded.
Yelp, which has more than 100 million unique visitors a month, expressly forbids paid reviews.
About 42 million reviews have been published on Yelp since its launch in 2004.