In his best-selling new memoir, restaurateur Keith McNally reveals that he had a steamy romance with an extremely famous Hollywood actress.

But he refers to her only as “X” and gives just a handful of clues about his high-profile paramour’s identity.

As far as Page Six can tell, the star in question is “X Files” actress Gillian Anderson.

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In his new memoir, Keith McNally says he had an romance with a Hollywood actress, but doesn’t name her. Clues suggest its “X Files” star Gillian Anderson.Gareth Cattermole/Getty Images

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McNally is the founder of Morandi, Balthazar, Minetta Tavern and other famed Manhattan eateries.Courtesy of Keith McNally

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McNally was on a break from future wife Alina Johnson during his romance with Anderson.Courtesy of Keith McNally
The Balthazar owner writes in well-received tome “I Regret Almost Everything” that the actress got in touch with him through a mutual friend after she saw a story about him in Vanity Fair in 1998.

He says that he didn’t know who she was despite “her starring in a TV show watched by millions.”

McNally writes that they got to know each other over the phone, and after a few calls, the LA-based star said she’d come and visit him in New York. (She surprised him by deciding to stay in his fourth-floor Soho walk-up, rather than a hotel).

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In the book he said “X” was on a show “watched by millions.”Getty Images

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He went onto have daughter Alice with Alina.Courtesy of Keith McNally
He also says that they went to see a Broadway play called “Art,” which dates her visit between 12 February 1998 and 8 August 1999. And he says that a picture of them together appeared in the New York Post.

The Post doesn’t appear to have published a photograph of McNally and an actress during the run of that play. But the legendary Rush & Molloy column in the Daily News did. (Perhaps McNally is misremembering which paper published the pic, or perhaps he intentionally transposed the papers in the book to help disguise the identity of “X.”)

‘”X-Files” star Gillian Anderson had a cozy dinner with British restaurateur Keith McNally on Saturday,” George Rush and Joanna Molloy reported, “Where’d they eat? Where else but McNally’s still-white-hot celebreteria Balthazar. McNally, who delivered the sexy redhead to her hotel after dinner, vouches that, ‘She’s only a friend. Really, we would have gone out the back way if anything was happening.’”

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He claims “X” screamed “f**k off!” down the phone when he ended it.Getty Images

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They had the affair in 1998.Courtesy of Keith McNally
Why downplay the romance? The answer may lie in McNally’s at-the-time off-and-on relationship with Alina Johnson, who he went on to marry in 2002.

“The morning [“X”] left New York, I scrubbed my apartment and called Alina,” he writes, “She was in a foul mood. She’d just seen a photo of my with X in the New York Post.”

(In some ways, the cloak-and-dagger act in the book is slightly odd, since McNally has already publicly revealed that he had a relationship with Anderson. “My girlfriend at the time asked me to ban Gillian Anderson, who had been a brief girlfriend of mine, [from my restaurants]” McNally told the Wall Street Journal in 2019, “Of course, I said no.”]

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The book is out now on Gallery.@keithmcnallynyc/Instagram
McNally, who also founded the Odeon, Morandi, Minetta Tavern and Schiller’s, among other famed Manhattan eateries, writes in the book that the first night that “X” arrived in New York, they didn’t have sex because she was tired.

“I thought spending the night together and not having sex meant the end of the relationship,” he writes, “No one told me that nothing turns women on more than not having sex.”

He writes that the next even he took her to dinner at Balthazar and went back to his apartment.

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Balthazar remains a hot spot.Bloomberg via Getty Images
“Few things impress a date more than taking her to a restaurant you happen to own,” he writes.

“Within ten minutes [of getting home] I was lying next to X’s naked body.”