Ratings for Brian Williams’ newscast fell sharply last Friday — two days after the “NBC Nightly News” anchor apologized for falsifying a story about flying in a helicopter that was hit by a grenade in Iraq.

Preliminary Nielsen ratings show that “Nightly News” lost 36 percent of its viewers in the key 25-to-54 demographic on Friday compared to its Monday to Thursday average last week, according to the TVNewswer website.

That’s twice the dip suffered by CBS and ABC, which saw viewership decline 17 and 16 percent.

Friday’s “Nightly News” was beaten by ABC’s “World News Tonight,” anchored by David Muir — which averaged 8.5 million viewers to a little under 8 million viewers for Williams, according to the Associated Press.

The Friday before the revelations about Williams came out, “Nightly News” beat “World News” by 400,000 viewers.

Williams announced over the weekend he was taking “several days” off from “Nightly News” (6:30 p.m./Ch. 4) and would be replaced during that time by Lester Holt.

He then canceled an appearance on “Late Show with David Letterman,” which was scheduled for this Thursday.