Travel advice website TripAdvisor is being investigated by the advertising watchdog over allegations that not all its reviews are genuine.
A formal complaint was lodged with the Advertising Standards Authority (ASA) over TripAdvisor's claim that it publishes "reviews you can trust".
The allegation is that some of the reviews may not be "real or genuine", an ASA spokesman said.
The complaint was lodged by KwikChex, a brand reputation management company.
TripAdvisor says its website attracts 45 million unique visitors every month and contains more than 50 million reviews.
Many of the reviews are posted by site users and help advise customers about hotels and restaurants.
The Press Association, photo by Mostad