Created by Brothers and Sisters, which had been appointed to the account last year, the ad shows a man using various objects to hide his discoloured, crooked teeth in order to take a selfie.
A voice over then says: "Love your teeth. Ask your dentist about Denplan", while the text on screen reads: "#doitforyourselfie".
The Advertising Standards Authority received 11 complaints about the ad, which aired last year.
When contacted by the ASA, Denplan said its ad was intended to promote its preventative dental care plans and encourage consumers to take a proactive approach to their oral health.
Denplan said its ad did not suggest that it would rectify teeth like those featured, but encourage preventative actions. It noted that the actor in its campaign was wearing a prosthesis that would not require any purely cosmetic interventions.
Because cosmetic dentistry was not available under Denplan, the standards watchdog concluded the ad was likely to mislead viewers.
In its ruling, published today, the ASA said: "We considered that viewers were unlikely to know exactly which treatments the man required to make his teeth look good again, and were likely to assume, given that his teeth were stained yellow, crooked and cracked, that some of the treatments would be cosmetic.
"Because cosmetic dentistry was not available under Denplan, we concluded the ad was likely to mislead viewers."
The watchdog ruled that the video must not appear in its current form again.