Channel 5 documentary The Sex Business breached broadcasting rules, watchdog Ofcom has said.
The regulator ruled on the three-part series, which had programme titles Pain For Pleasure, Trans On Demand and Orgasms For Sale and investigated the lives of sex workers in Britain.
An Ofcom spokeswoman said: “Our investigation found these programmes broke our rules by including images of extreme, graphic sexual activity.
“These were far more explicit than viewers would be likely to expect in a programme broadcast an hour after the watershed.”
The programme, which aired at 10pm last year, featured interviews with sex workers and “images of strong, extreme and graphic sexual activity”.
There was “insufficient masking” and close-up shots resulted in “sexual material that was extreme, graphic, prolonged at times and prominent”, Ofcom said.
It cited data showing how many children may have been watching, with 30,500 aged between four and 15 for the second episode.
The programme breached rules on post-watershed explicit sexual content and offence.
A Channel 5 spokesman said: “The Sex Business is a programme of public interest, offering a unique insight into an industry employing around 100,000 people in the UK.
“We note that Ofcom considered this was a serious observational documentary and that the inclusion of sexual content clearly supported the editorial purpose, presenting the lives and experiences of the workers featured in the series.
“Although there were very clear warnings as to the explicit nature of the content both before the programme and every break part, we acknowledge the findings from Ofcom and will reflect on the decision before repeating any of the episodes in question.”