NBC Nightly News anchor Brian Williams is “cring­ing” with embarrassment because his 23-year-old daughter Allison is starring in a raunchy, sexy TV show, say sources.

While publicly supportive of Al­lison’s showbiz ambitions, the newsman is red-faced over her participation in the HBO sitcom “Girls” because she performs in a sex scene in the very first episode.

Allison – the older of 52-year-old Williams’ two children by wife Jane – landed the plum role of Marnie in the much publicized comedy, which pre­mieres April 15.

Her dad, who’s witnessed all sorts of horror and heartbreak in his news career, reportedly had to close his eyes while watching his daughter’s charac­ter simulate a solo sex act in an advance screening of the first episode.

“Poor Brian got the shock of his life watching the scene,” revealed a friend. “Allison was beaming with pride that she has a starring role on a TV show, but all Brian saw was his little girl getting down and dirty.

“Allison noticed her dad squirming and told him, ‘You know it’s only acting! That’s a character, not me on the screen.’ But Brian told her, ‘It sure looks like you – don’t you care what people are going to think of you?’

“Brian envisioned her working on some English period piece, not pleasuring herself on-screen!

“It makes him cringe that old friends are now coming up to him and saying, ‘Your daughter – wow!’”

Allison graduated from Yale University in 2010 with a dream of becoming an actress. And “Girls” producer Judd Apatow – the director behind the movies “The 40-Year-Old Virgin” and “Knocked Up” – gave her a big break by choosing her for the cast.

“The show is sure to be a big hit,” said the source. “But while Brian wants his daughter to be success­ful, this part is threatening to start a family feud.”