Watchdogs will launch an inquiry into Catherine Tate’s comedy special after complaints by viewers that it was the most offensive programme ever broadcast by the BBC on a Christmas Day.

Viewers complained of excessive use of the “f-word” by Tate’s foul-mouthed character Nan. A sketch depicting a Northern Irish family as terrorists prompted accusations of bigotry.

The sketch show attracted 6.4 million viewers to BBC One at 10:30pm on Christmas night. The BBC defended the show, describing Tate as a comedy genius. But Ofcom said it would examine the programme after receiving complaints about offensive language. The inquiry will ask whether the programme was appropriate for Christmas night, when many children would be watching.

Viewers complained that the programme, which followed the more placid To The Manor Born, began with an avalanche of swearing from Nan Taylor. Kathy Burke, playing her daughter, embarked upon a swearing competition with Nan.

The representation of a family in Northern Ireland receiving Christmas presents attracted complaints that Tate was exploiting lazy stereotypes. The grandmother opens her present to find a balaclava, which she puts over her head. Her husband receives a knuckleduster which he excitedly uses to punch a chair. The mother’s gift is an apron with a balaclava-clad terrorist and the words “Remember Everything, Forgive Nothing”. A gay son is handed a chocolate penis.

One viewer wrote on the BBC online message board: “What had the contents of this to do with Christmas? Crude language just for the sake of it – and being repeated time and time again.” Another commented: “Maybe the lowest point for BBC One on Christmas Day... ever.”

Ms Tate admitted that the language might have got out of hand. “I don’t know how this Christmas special got so depraved because it isn’t what I set out to do,” she told Radio Times. The sketch between Nan and her daughter required a “climactic aspect when you’re topping each other” with greater feats of swearing.

A spokesman for the BBC said: “Catherine Tate creates characters who are so over the top as to be almost cartoon-like and this is where her genius lies. Her comedy is never meant to offend any viewer and is always based on satire and grotesque exaggeration.” The Nan character’s foul language was “fundamental to what makes her funny” and the show was preceded by a warning that it contained strong language. The BBC received about 100 complaints through telephone and internet message boards.

Tate, 39, was nominated for an Emmy and won a British Comedy Award for her show, which began on BBC Two. She has no plans to make a further series and said that she wanted to retire the show before the characters got stale. She will revive her role as David Tennant’s assistant in the next series of Doctor Who.

A spokesman for Ofcom said: “We have received complaints about offensive language and content in the Catherine Tate Christmas Show and we will look into the matter.”

Tate showed her determination to move on by sending her most famous creation, Lauren, the schoolgirl whose catchphrase was “Am I bovvered?”, in the special episode, on a fatal canoeing trip.

Bovvered?

“Am I bovvered? Look at my face. Is my face bovvered?”
Lauren Cooper

“What a f***in’ liberty!”
Joannie “Nan” Taylor

“The dirty robbing b******s!"
Janice and Ray

“Who, dear? Me, dear? Gay, dear? No, dear!”
Derek Faye

Viewer response on BBC Points of View message board:

— “What had the contents of this to do with Christmas? Crude language just for the sake of it – and being repeated time and time again. The once-proud BBC has dropped into the gutter.”

— “Swearing I can take to a point but when it goes on and on it just gets boring. Maybe the lowest point for BBC One on Christmas Day . . . ever.”

— “Stereotypical Asian jokes, gay jokes. Old hat in the Seventies.”

— “Crude . . . tasteless . . . foul language . . . cheap. Where does CT go from here to get a laugh?”

— “They must have used the F-word, which offends me greatly, at least once in every sentence, and it is not necessary.”

— “The CT shows were fresh, original, funny, with excellent ideas. The Christmas thing depended solely on bad language and lewd allusions to some sexual activities. Out of steam. Sell-by date well gone.”

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