The Press Complaints Commission has upheld a complaint against the magazine FHM for publishing a topless photograph of a 14-year-old girl without consent.
The girl's parents said the publication of the photo, taken in 2005, was a serious breach of her privacy.
The Press Complaints Commission said that publishing the picture, sent in by her boyfriend, was a significant breach of its guidelines.
It upheld a complaint by her parents after the picture appeared in a gallery of mobile phone pictures sent in by readers in the magazine's April edition.
The PCC said the decision to publish the image of the girl without adequately establishing consent represented a serious intrusion into her private life.
"The magazine had clearly not taken any sort of adequate care to establish the provenance of the photograph and whether it was right to publish it.
"This would have been the case regardless of how old she was, but the commission was particularly concerned about the impact on the girl in light of her youth," it added.
In its ruling the PCC said: "It should have been much quicker to recognise the damage that publication would have caused the girl, and offered to publish an apology or take other steps to remedy the situation."
FHM said it had been informed that the girl was in a relationship with the person who submitted the photo and had made no further enquiries.
The lads' mag said it receives 1200 photographs each week from, or on behalf of, women posing topless or in underwear.
It said the teenager looked older than 14 and had no reason to believe the image was taken without consent.
A spokesman for FHM said: "We regret any distress caused to either the girl or her parents.
"When the picture - a posed, topless shot - was submitted to us for publication it appeared to be of a much older girl taken by a male friend.
"The information with which we were provided also suggested this was the case."
FHM has since introduced new measures to prevent a similar breach of the PCC code.