This year, Richard Desmond's Channel 5 rebooted the Big Brother reality series after it had been dropped by Channel 4.
However, a complainant contacted Ofcom about two episodes of the show aired in late September, which featured housemates using the words "f**k" or "f**king" just after the 9pm watershed.
On September 23, housemate Rebeckah was heard saying, "Are you f**king crackers?" 11 seconds after the watershed, and again less than six seconds into the programme.
The episode broadcast on September 30 featured Harry shouting, "Stay the f**k out of other people‟s business" 18 seconds after the watershed, and another housemate said "f**king" 31 seconds after the watershed in the same broadcast.
Ofcom therefore decided to launch an investigation into the episodes under rule 1.6 of the broadcasting code, which stipulates that there must not be an "abrupt" transition to more adult content just after the watershed
Channel 5 said that it "regretted" that a viewer had been offended by the language, but denied any suggestion that the transition was "unduly abrupt" after 9pm.
Channel 5 said that it had "acted reasonably and responsibly and made its decision to include strong language in the pre-titles only after serious consideration and due regard was given (at senior levels) to the relevant Ofcom rules".
The broadcaster said that there was an "unambiguous warning" aired at the start of each episode warning of strong language, and claimed that the swearing was intended to reflect the "heightened tensions" between the housemates.
It further referred to previous Ofcom rulings on the use of the word "f**k" in live pre-watershed episodes of Big Brother when it was aired on Channel and E4, in which the regulator opted against recording a breach of the code.
Channel 5 said that these earlier findings indicate that Ofcom had "distinguished between audience expectations of Big Brother pre-watershed and post-watershed, the latter including strong language".
But Ofcom said in its latest Broadcast Bulletin that the previous cases raised by Channel 5 referred to live Big Brother broadcasts, which were later accompanied by on-air apologies to viewers.
In contrast, Ofcom said that Channel 5's case involved the offensive language being "deliberately selected and inserted into two pre-recorded sequences".
"Ofcom did not consider there was sufficient editorial justification to include repeated use of the most offensive language in these programmes so soon after the watershed," said the regulator.
"The two uses of the word "f**k" or "f**king" in each programme in the period directly after the watershed did in Ofcom's view constitute an 'unduly abrupt' transition to more adult material at the watershed. Rule 1.6 was therefore breached."
Channel 5 caused controversy before launching the revamped Big Brother format by ruling out the popular live stream of the show that was offered previously by Channel 4.
In August, Channel 5's director of programmes Jeff Ford said that people had "moved on" from the Big Brother live feed, despite continuing criticism from viewers.