It was September 1989, and Princess Diana was staying at her ancestral home of Althorp in Northamptonshire, England, when her fury for her stepmother boiled over.
The royal, then 28, berated Countess Raine Spencer, who had married her father, Johnnie, 13 years earlier. Then she pushed her so hard, she fell down the stairs.
“[Raine] was badly bruised and was dreadfully upset,” Raine’s former personal assistant, Sue Howe, explains in a revealing new documentary. “It was a cruel and heartless thing to do.”
The fraught relationship between troubled Diana and larger-than-life socialite Raine is the focus of the film “Princess Diana’s ‘Wicked’ Stepmother,” premiering on the Smithsonian Channel Monday.
Both women are dead — Diana was killed in a car accident in 1997, while Raine succumbed to cancer in 2016 — but their rivalry and surprising eventual friendship is fascinating.
“They were strong to the core — survivors,” Diana’s former butler, Paul Burrell, told The Post. “They suffered adversity and tragedy which brought them together in the end.”
Raine, the daughter of flamboyant British romance novelist Barbara Cartland, first entered Diana’s universe when dating her father, Earl John Spencer, following his 1969 divorce from Diana’s mother, Frances Shand Kydd.
Together with her younger brother, Charles, and two older sisters, Jane and Sarah, the young Diana gave this interloper the caustic nickname “Acid Raine.”
“They were used to having their father to themselves, and when Raine came along it was a disaster,” said royal biographer Penny Junor. “And Raine was not sensitive about the way she handled matters, [such as] changing Althorp because she had no sentimental attachment to it.”
She sold precious antiques to fund renovations of the dilapidated stately home and imposed her flashy taste on the decor. As Burrell explained: “Diana said she made it look like Disneyland.”
In 1978, the outrageously bouffant-haired Raine refused to allow Diana and her siblings to visit their father after he suffered a near-fatal stroke, causing further friction.
“She thought it would be better if he had peace, rest and no agitation,” recalled Junor.
John recovered to proudly walk Diana down the aisle at her wedding to Prince Charles in 1981. Raine, meanwhile, was banished to a seat at the back of St. Paul’s Cathedral.
As Diana’s marriage to Charles went into free fall, the princess used her stepmother as an outlet for her anger and frustration. “It was pretty full-on war,” said Junor.
When John died in 1992 — the same year Diana separated from Charles — Raine was unceremoniously kicked out of Althorp by her stepchildren. She went to live in the London townhouse her husband had left her in his will.
According to Burrell, things took a surprising turn during the last three years of Diana’s life, as the two women actually reconciled. Raine, newly married to French Count Jean-Francois de Chambrun, started writing to her former stepdaughter.
“The princess did a complete U-turn and invited Raine and her husband for lunch,” said Burrell.
In the documentary, Peter Constandinos, Raine’s hairdresser and friend, recalled what the then 70-something socialite told him about that lunch.
“Diana said to Raine: ‘I have to thank you. I know you loved my father deeply, and I have to be grateful for all the years of happiness you gave him,’ ” he says. The two hugged, and their reconciliation began.
“Diana mellowed as she got older, and they were quite similar characters,” Junor said.
Soon, the women were regularly spotted around London having lunch. Raine showered Diana with flowers, chocolate and gifts such as Hermès scarves.
But Diana may have been using Raine as a pawn.
Burrell told The Post that the princess would arrange for newspaper photographers to snap pictures of her embracing Raine in public — a move orchestrated to spite Shand Kydd, from whom Diana was then estranged.
Tragically, the women’s newfound friendship came to a premature end with Diana’s death in Paris on Aug. 31, 1997, at 36.
Raine visited Burrell at Kensington Palace the night he returned from France with Diana’s body. “She cried and held my hand and said, ‘What are we going to do now?'” said the butler. “She was devastated.”
Raine died Oct. 21, 2016, at 87, after throwing a “farewell” dinner for her closest pals.
As her friend Geordie Greig, now editor of the Daily Mail, says in the documentary: “Was she a force for good? Was she the evil stepmother? Or was she the femme fatale? She was all of those things. What she wasn’t was dull.”
