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The out-of-wedlock granddaughter of a late British art collector is poised to claim half-ownership of a $1 billion collection of paintings, sculptures, tapestries and other objects that were left to New York University in 1994, Page Six has learned.

The school has called the treasure trove — amassed by Sir Arthur Acton and bequeathed to NYU by his son, Sir Harold Acton — part of the “most magnificent gift” ever to an American university. But a July 17 ruling in a decades-long court battle in Italy said that DNA proved Acton also left another heir — the late Liana Beacci, daughter of Arthur’s mistress, Ersilia Beacci.

And Liana’s daughter told Page Six she’s “100 percent certain” the court will also posthumously award her mom an equal share of the more than 6,000 artworks, French dresses and Baroque furniture.

“It’s a great satisfaction,” Italian Princess Dialta Alliata Lensi Orlandi said of the recent ruling.

“It’s about 25 years that NYU has fought us not to have the paternity of my mother recognized. Finally, the respect and consequent rights that are owed to my mother for much too long have been given to her.”

The Acton Collection decorates the interior of the 14th-century Villa La Pietra, the main house on a sprawling, 57-acre estate overlooking Florence that in 1903 became the home of Acton and his wife, Chicago banking heiress Hortense Mitchell Acton, who paid for the spread.

They left the estate to their son, Sir Harold Acton, a poet and historian who was friends with famed writer Evelyn Waugh, and inspired the Anthony Blanche character in Waugh’s masterpiece, “Brideshead Revisited.”

Harold, who was also friends with artists including Pablo Picasso and writers including D.H. Lawrence and Graham Greene, died at the estate at age 89.

He left no immediate family members behind, and his will gave the estate — which also includes four other historic villas, gardens and olive groves — to NYU, which turned it into an appointment-only museum and “study abroad campus” for its students.

The school’s website calls Villa La Pietra “the jewel in the crown of New York University’s global program.”

Liana sued NYU over the inheritance but died in 2000, before it could be resolved. Her daughter, who is married to a wealthy Italian art collector, carried on the legal battle.

“I don’t need any money at all — we live in great luxury,” she said, adding that the fight was “all about principle to me.”

Orlandi also said she wasn’t interested in the estate, only the art that belonged to her grandfather. NYU spokesman John Beckman said the school was still reviewing the court ruling and hadn’t yet decided whether to appeal. “Importantly, this decision relates to the issue of paternity and has no bearing on the inheritance claims, which remain before the court,” he said.

“With respect to inheritance, many crucial legal and factual issues remain to be addressed including, significantly, the fact that Villa La Pietra was purchased by Hortense Action — the putatively betrayed spouse.

“We remain confident that in the end, the courts will not disrupt the vibrant cultural and educational institution that NYU has created at Villa La Pietra in accordance with Sir Harold Acton’s wishes.”

Additional reporting by Bruce Golding