The website carried statements such as "more than 50 million honest travel reviews and opinions from real travellers around the world" and "reviews that you can trust", the Advertising Standards Authority said.
But the watchdog upheld complaints that the US-based company -- which claims to be the world's biggest travel site -- did not verify the reviews to the extent that it could guarantee they were all genuine.
It said that claims that all reviews on the site were from actual travellers were "misleading".
"We told TripAdvisor not to claim or imply that all the reviews that appeared on the website were from real travellers, or were honest, real or trusted," it said in a statement.
TripAdvisor said its fraud detection systems were "advanced and highly effective" but that it was "not practical" for them to screen all reviews manually.
A key complaint was made last year by an online reputation management firm called KwikChex.com, which said it had spoken to thousands of hotels claiming to be affected by malicious reviews, the Guardian newspaper reported.