Killer real estate heir Robert Durst is trying to bury a damning letter from a former cop that blames him for the death of his wife Kathie Durst, according to new court filings.

“I think this is the beginning of proving Durst killed Kathie,” a source close to the Long Island case told The Post.

Durst’s lawyers have asked a Nassau County judge to seal a handwritten, Feb. 12 letter from ex-cop Edward Wright to Robert Abrams, the attorney for Kathie’s sisters.

The three siblings — Carol Bamonte, Mary Hughes and Virginia McKeon — have a $100 million wrongful death case pending against the scion over the 1982 disappearance of Kathie.

They named Wright as a defendant in the civil case claiming Durst’s late father, real estate mogul Seymour Durst, paid Wright to perform a “shadow investigation” that kept his family abreast of the NYPD probe into Kathie’s disappearance.

Wright, who is in his 80s, tried to answer the allegations on his own without a lawyer earlier this year.

His statements in the new letter “inculpate Durst in the disappearance and murder of Kathleen McCormack Durst,” Abrams says in the Nassau County court filing.

Durst’s lawyers learned of the communication recently and rushed to court Tuesday to seal the evidence. They argued that the letter’s contents are protected by attorney client privilege because the scion’s former defense counsel Nicholas Scoppetta had hired Wright.

Wright has previously said that the NYPD “knew or should have known” that Durst had repeatedly lied about “his involvement in Kathie’s disappearance and murder.”

Durst remains the only suspect in her death and is currently being tried in Los Angeles for killing his friend Susan Berman in 2000 to prevent her from squawking to law enforcement officials about his role in Kathie’s disappearance.

He was arrested for Berman’s murder in March 2015 — a day after the airing of the finale of HBO’s docu-series “The Jinx,” in which Durst is caught muttering to himself on a hot mic, “What the hell did I do? Killed them all, of course.”

In 2001, the millionaire — who at the time was living as a cross-dressing woman named Dorothy Ciner in Galveston, Texas — was charged with murdering his elderly neighbor Morris Black.

He had admitted to killing Black and chopping up his body, but a jury acquitted him of homicide charges after he argued the shooting was in self-defense.

The 75-year-old has otherwise denied that he was involved in either Berman’s or his wife’s death.

In the new court filings Abrams argues that the attorney client privilege between Wright and Durst’s now dead defense lawyer no longer exists.

But Durst’s current counsel Joshua Siegel says the note must remain private.

“The letter is privileged and otherwise inadmissible, and we hereby demand that you do not file, publish, disclose or otherwise seek to use the letter for any purposes whatsoever,” he wrote in a May 3 communication with Abrams.

He threatened Abrams with “civil damages, sanctions and attorney’s fees” if he tried to make the letter public.

Nassau County Court Justice Roy Mahon temporarily sealed the missive Tuesday, pending a full hearing on the matter later this month.

Abrams declined to comment and Siegel did not immediately return messages.