The Advertising Standards Authority (ASA) has banned a billboard that showed Santa Claus vaping, because the advert could "appeal" to under-18s.
It also ruled against two similar billboards which appeared in the UK in December, one involving an elf, the other a gingerbread man.
The three adverts were promoting a website called The Vapes, which claims it never meant to target children.
But the ASA argued the use of cartoons made the characters "child-like".
It's illegal in the UK to advertise e-cigarettes in a way "likely to appeal particularly to people under 18 years of age".
Researchers at Cardiff University say children are nearly twice as likely to try e-cigarettes than tobacco.
As well as being dressed in denim, the Santa featured on the billboard had his sleeve rolled up to reveal a "Rudolph" tattoo on his arm.
The image had "All Santa wants for Christmas" written alongside it.
Three people complained.
Fischen Medical Limited, the company which runs The Vapes, claims it deliberately tried not to use "traditional seasonal imagery".
It says it was trying to "appeal to the spirit and culture of a target audience of adults, with an 'off the wall', fun and alternative advertising campaign".
It also argues the characters used had a "sinister appearance" that would put children off.
But the ASA upheld the complaints, arguing that "all three characters were drawn in a cartoon-style, and we considered that they were child-like rather than adult in appearance.
"They were all characters that children would recognise, and we considered that those characters were likely to appeal to them more than they would to adults."
It has told the company not run the adverts again and to make sure it complies in future.