The three-part series “Quiz,” premiering Sunday (10 p.m. on AMC), chronicles the real-life cheating incident that nearly destroyed Britain’s version of “Who Wants to Be a Millionaire” in 2001 — when Major Charles Ingram (Matthew Macfadyen, “Succession”) and his wife, Diana (Sian Clifford, “Fleabag”), were accused of masterminding a series of in-studio coughs to help Charles win the show’s 1 million pound grand prize.

The couple and their alleged “accomplice,” Tecwen Whittock (the cougher, played by Michael Jibson), were put on trial after the show’s producers noticed afterward that Whittock, sitting directly behind Ingram in a “fastest finger” chair, coughed as if on cue whenever Ingram seemed to be stumped on an answer.

It was also strange to them that Diana and her brother, Adrian (Trystan Gravelle, “The Terror”), had both appeared in the hit ITV show’s “hot seat” opposite host Chris Tarrant (Michael Sheen) before Ingram’s appearance.

The series, directed by Stephen Frears (“A Very English Scandal”), will air a new episode each Sunday through June 14.

“It was one of those shows that you watched night after night and it was a huge cultural touchstone when it first started,” Macfadyen tells The Post about the UK “Millionaire,” which premiered in 1998 (one year before ABC’s prime-time version hosted by Regis Philbin). “It got up to 19 million viewers a night at its peak and it was a big deal.

“It was really successful because there was no time limit [for the contestants to answer questions] … no clock or countdown. It was much more theatrical and that was the key to its drama — the pauses.”

Ingram was only the second person to win the grand prize; his drive to the top was rife with the aforementioned dramatic pauses, ear-scratching, twiddling his wedding ring and his habit of changing his mind before almost every “final answer,” much to Tarrant’s (and the viewers’) amusement.

There’s plenty of built-in ambiguity in “Quiz” regarding Ingram and company’s guilt or innocence — which spurred a book (“Bad Show: The Quiz, the Cough, the Millionaire Major”) and a 2017 theatrical play (also called “Quiz”) from which the series is adapted.

“It’s a really peculiar thing to play this man,” says Macfadyen who, along with Clifford, met the Ingrams briefly on their last day shooting the “Millionaire” scenes. “It’s not a strict impersonation of him but a retelling of the story. It wasn’t like I was copying his every movement … it was just my impressions of him. It’s an odd thing to factor into playing someone who’s been through this — and has a cloud of doubt hanging over them. Their lives were changed irrevocably.”

Macfadyen says that both he and Clifford waffled over whether the Ingrams were guilty or innocent.

“Every day Sian and I would see each other in makeup we’d go, ‘Right, what do you think? They’re guilty.’ The next day we’d say, ‘No, I think they were stitched up and were innocent.’ We flip-flopped, and at the end [of the shoot] we were agnostic. Even now I don’t know what to think and I sort of don’t mind. By the end, I was more interested in them and what they went through. We can’t know what really went on.

“You play these little incremental moments as truthfully as the script allows,” he says. “What’s fascinating is what the [viewing] audience projects onto you … how we suit things to fit our narrative and confirmation, our prejudices … how we remember things in certain ways. Their lawyer [played by “Peaky Blinders” star Helen McCrory] talks about memory and how it’s unreliable.”

ITV yanked Ingram’s winning “Millionaire” episode and it was never televised; parts of it were used during the trial and as part of a later ITV documentary about the scandal.

“People think they saw it, but they haven’t,” says Macfadyen. “What they saw is ITV’s edited version that was made for the cops [during the trial]. I watched that … to get a feel for him and how he moved. [The series] is nicely nuanced; I really believe the [‘Millionaire’] producers were passionate about the show and you feel for [producer] Paul Smith [Mark Bonnar] — he’s heartbroken that people are cheating on him and he takes it personally when he thinks he’s being duped.”

And, Macfadyen says, he got a taste of what the average “Millionaire” contestant experienced in the hot seat.

“I feel like I’ve done it,” he says. “There was no acting required, certainly not on the first day on the ‘Millionaire’ set. They built an exact replica with the lighting, sound effects, and there was an audience there.

“It was quite terrifying, which helped.”