Media watchdog Ofcom received a complaint about offensive language in the optional subtitles of a repeated episode of the British TV series, which aired on June 12.

The programme included two instances of the word "f*****g" despite the show's audio not using any offensive language.

On Monday, Ofcom ruled the programme had breached its rule that prohibits the "most offensive language on television before the watershed" as it aired at 10.30am on TV channel Drama.

UKTV Media, which holds the licence for Drama, said the episode was "incorrectly subtitled" and apologised for the incident.

It added the subtitling of the show is outsourced to Red Bee Media and the subtitler, who had not followed protocols, had been "disciplined and withdrawn from subtitling duties until they have been through a thorough retraining process with their line manager".

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The UK regulator said that while it took into account UKTV had not experienced the issue before and took action to address it afterwards, there were no mitigating factors such as on-air apology.

Ofcom ruled the broadcast was in breach of rule 1.14 of the code, which prevents offensive language before the watershed.

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A spokesperson from UKTV said: "This subtitling error occurred due to human error and we apologise for any offence caused."

Drama is a television channel which specialises in repeats of well-known British TV drama series, including Casualty which usually airs on BBC One.