Comcast Corp.’s NBC television network desperately needs more prime-time hits. But did it let one slip away?

NBC passed on current cultural favorite “Downton Abbey,” produced by NBCUniversal’s Carnival Films studio in London, believing that American audiences wouldn’t have the appetite for a very British historical drama set in a country manor in Edwardian England.

Instead, “Downton Abbey” found a grateful home on public television, as part of the “Masterpiece” lineup. Spoofed on “Saturday Night Live” this year and rated No. 2 nationally at 9 p.m. Feb. 5, the night of the Super Bowl (aired by NBC), “Downton” has been a stunning success for PBS.

On Feb. 19, 5.4 million viewers watched the season finale, capturing a 3.5 national Nielsen rating and making it the highest-rated PBS show since the premiere of Ken Burns’ “National Parks” in September 2009, PBS says.

“They may have second thoughts about letting it go to PBS,” Brad Adgate, senior vice president of research at Horizon Media, said of NBC. “Downton Abbey” probably never would have rated No. 1 in prime time for NBC, but it could have built a solid viewership and brought other benefits to the network’s tarnished reputation.

In an era of reality shows and poorly staged TV dramas, “Downton Abbey” “gets a lot of critical acclaim, so it’s a prestigious show,” Adgate said, noting that the series appeals to a sophisticated and urban, though older, audience. Facing aging demographics, network television has been seeking shows that appeal to younger audiences.

NBC has improved its prime-time showing recently with “Sunday Night Football,” the Super Bowl and “The Voice,” Adgate said. But the network still has big holes to patch. The 10 p.m. Thursday time slot is one of them. Between 1981 and 2009, NBC aired just three shows in that slot: “Hill Street Blues,” “L.A. Law” and “ER.” By contrast, the network has carried three shows in that slot since September: “Prime Suspect,” “The Firm” and “Awake.”

An NBC official said last week that the network had been delighted at the success of “Downton Abbey” for its sister company and that the decision not to air the show in the United States was made under a previous NBC administration.

Comcast acquired control of NBC Universal Inc. in early 2011 and vowed that it would revive the sagging fortunes of the NBC TV network, which suffered under former owner General Electric Co. The Philadelphia cable company replaced many top entertainment honchos, and now former Comcast executive Steve Burke heads the news and entertainment conglomerate. Comcast officials have said that if they can fix NBC TV, there is potential to earn hundreds of millions of dollars in profit.

But even current NBC executives could have overlooked “Downton Abbey’s” potential, NBC acknowledged. The official said it was hard to imagine any network — including PBS — thinking “Downton” would become a hit.

As a practical consideration, there seems to be doubt that “Downton Abbey” could sustain 22 episodes over a TV season, which is necessary for a U.S. network. Seven “Downton” episodes aired on PBS this year.

That said, NBC seems to be reconsidering the accepted network TV wisdom. Bob Greenblatt, the new head of entertainment for NBC, developed the period dramas “The Tudors” and “The Borgias” at the cable channel Showtime. While at Showtime, Greenblatt also developed a series about the Vanderbilts with the writer Julian Fellowes, creator of “Downton Abbey.”

NBC is experimenting with pilots for a Western, an epic pirate tale about Blackbeard, and a Dracula story set in 19th-century England. Greenblatt, the NBC official said, knows the virtues of such shows — they just have to be broad enough to work for broadcast TV.

Downton Abbey executive producer Gareth Neame said in a phone interview last month that he was filming a third season, and that though the series is a historic drama, it’s fast-paced and told in a modern way. One model for the series was “The West Wing,” he said.

Though a Brit, Neame boasts a Hollywood pedigree. His grandfather, Ronald Neame, had a four-decade career as a cinematographer, producer, screenwriter and director. His most acclaimed credit was as director of “The Poseidon Adventure,” released in 1972.

Gareth Neame believes the success of “Downton Abbey” could lead to a more receptive audience for the historic-drama genre among U.S. TV executives.

As for PBS, he said, “they have always been there for British producers. They don’t have the biggest checkbook, but they are consistent.”