Three viewers complained about the use of the word in a trailer for 'Deadwood', which is based on a real American frontier town in the late 19th century.
Two viewers were watching programmes such as 'Star Trek' and 'Law & Order', which do not contain this kind of language, and argued they felt it was inappropriate for trailers to use this language around such programmes.
Ofcom ruled in favour of the complainants saying that the trailers were not appropriately scheduled due to the series being unfamiliar to many viewers and because they were shown during programmes appealing to a wide variety of viewers.
Two versions of the trail were produced, one without the offensive language and the other finishing with the phrase "Run for your fucking life".
Sky said it believed the relatively small number of complaints demonstrated that it had taken sufficient care and the trailers were scheduled appropriately.
The earliest trailer, at 9:55 pm, had been shown immediately after 'Miami Slice', a programme about plastic surgeons; the second was shown at 10:50pm, during an episode of 'Law & Order'; and the third was shown after midnight during 'Star Trek: The Next Generation'.
Trailers during this late-night screening of 'Star Trek: The Next Generation' were shown on only very few occasions and always between the hours of midnight and 2am.
Ofcom said: "We acknowledge that Sky had taken care to schedule these trails away from times when younger viewers would be watching."
The broadcasting regulator went on to say: "As with advertisements, trails come upon viewers unexpectedly and there is little opportunity to immediately understand the context in which any swearing is used."
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