Numerous ads for the comparison website Confused.com, including a TV version featuring the actor James Corden, have been banned for unsubstantiated claims about the amount of money customers could save.
The Google, email, TV, website and press ads all claimed the website is "No.1 for car savings", leading to a complaint from rival Gocompare.com that the message gave the impression that consumers would save more money with Confused.com than its competitors.
Confused.com said the ads did not claim they offered greater savings on individual products, and the No.1 claim included a qualification clarifying that it was based on "opportunities to save".
The Advertising Standards Authority (ASA) said consumers would generally understand the No.1 claim to mean that they were likely to save more money by using Confused.com.
It said: "Because the market research provided did not support those interpretations of the ads, we concluded that they were likely to mislead."
The ASA ruled the ads must not appear again in their current form, adding: "We told Confused.com not to state or imply in future that consumers were likely to save more money by using their site instead of their competitors' when buying car-related products unless it could be substantiated."
A Confused.com spokeswoman said: "In response to the recent ruling by the ASA, Confused.com has amended its adverts accordingly to remove the No.1 messaging."