The former London apartment of Princess Diana’s stepmother, the late British socialite Countess Raine Spencer, has hit the market for $10.5 million.

Growing up, Diana and her siblings were not fond of their stepmother — and referred to her as “Acid Raine” and often sang “Raine, Raine, go away!”

Before that, in 1948, Spencer married the Hon. Gerald Legge, the heir to the Dartmouth Earldom — and the couple had use of two homes in Mayfair. One is this two-bedroom apartment at 47 Grosvenor Square, which is now up for sale, and a townhouse at 12 Chester St.

Spencer had two sons with Legge.

When Spencer became a Westminster City Councillor in 1955, the two Mayfair properties were invaluable — with the 47 Grosvenor Square apartment useful as her office and political base, while the Chester Street townhouse served as the main family home.

Spencer eventually divorced Gerald, but Mayfair remained her home.

When she married her second husband, Diana’s father John Spencer in 1976, one of her first acts was to persuade him to buy a first-floor apartment in adjacent 48 Grosvenor Square, a purchase organized by real estate broker Peter Wetherell, the founder and executive chairman of Wetherell.

“With its iconic address and garden square views this Mayfair apartment at 47 Grosvenor Square provides a discerning buyer with a unique opportunity to buy a blank canvas shell unit and refurbish and modernise the property to create a beautiful new residence,” Wetherell told The Post in a statement.

“Once refurbished and interior designed, the apartment would serve as an excellent long-term investment and London residence.”

Interior photos show what the home would look like reimagined.

The apartment is located on the fifth floor and occupies 2,145 square feet.

“Despite its illustrious history, the property is currently an empty shell,” the listing notes, adding that the interiors have the opportunity to be transformed into a “magnificent new residence.”

Currently the home offers two bedrooms with dressing areas and ensuite bathrooms.

There is also an expansive lobby with two separate living areas and a dining area with a built-in cocktail bar that can seat up to eight people.

The family kitchen/breakfast room has a separate pantry-slash-utility room. From the kitchen, there’s a separate back-of-house staff corridor that connects the kitchen to the entrance lobby to assist staff circulation and hospitality provision.

In 2019, the film “Princess Diana’s ‘Wicked’ Stepmother,” premiered on the Smithsonian Channel.

In the documentary, it was claimed that in 1989, when Diana was 28, she had been staying at her ancestral home of Althorp in Northamptonshire, England, when her fury for her stepmother boiled over.

The royal berated Countess Raine Spencer — 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, says in the documentary. “It was a cruel and heartless thing to do.”

Although Diana endured a turbulent relationship with Spencer, their rivalry turned into a friendship.

“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 passed away on Oct. 21, 2016, after losing her battle to cancer at the age of 87.