Top Gear host Jeremy Clarkson breached BBC guidelines by comparing a Japanese car to people with growths on their faces.
A report by the the BBC Trust's editorial standards committee (ESC) found he "strayed into an offensive stereotypical assumption" with the scripted remarks during an episode of the hit motoring show.
A member of the public complained about the "offensive" remarks after Clarkson likened the shape of a Prius campervan to someone with "a growth on their face" and referred to it as the "elephant car".
He also described it as "not a car that you could talk to at a party unless you were looking at something else".
The ESC said the show's audience enjoyed the presenters' "sometimes controversial and forthright views" but ruled this remark "played on a stereotypically negative reaction to facial disfigurement".
It said it "strayed into an offensive stereotypical assumption" and was "not editorially justified".
The report stated: "The committee accepted that the remark was exaggerated for comic effect, but believed that some viewers may find this particular remark, taken within the whole context of the exchange, to be a purposeless stereotype.
"The committee concluded that this programme was in breach of the guidelines on harm and offence as the exchanges about facial disfigurement were not editorially justified and did not meet generally accepted standards in the context of their portrayal of a disability."
It upheld the complaint.
Clarkson was cleared by Ofcom earlier this year of breaching the broadcasting code by watchdog Ofcom over the same remarks.