The 66-year-old, who has lived at the 30-room Windsor property with her Andrew Mountbatten-Windsor, 65, since 2008, is expected to move out of the luxury property within months.
King Charles, 76, has reportedly made clear his responsibility extends only to finding accommodation for his brother.
Ferguson lost her Duchess of York title and much of her earning power after becoming embroiled in the scandal surrounding convicted sex offender Jeffrey Epstein.
In an email written to Epstein in 2011, which was made public earlier this year, Ferguson apologised profusely for distancing herself from him after his 2008 conviction for procuring a minor for sex.
A source close to her said the email had been written “in panic” and without legal advice, explaining: “She feared Epstein would sue her for libel. She hadn’t committed any crimes – she was just hapless. If there was a Second World War bomb in Piccadilly, Sarah would step on it and set it off.”
Socialite and party planner Liz Brewer told The i Paper that Ferguson had always been “very personable” but admitted she was currently unwelcome at high-society gatherings.
She said: “People, unfortunately, are not brave enough to make their own minds up. She’s in a very difficult position.”
Ferguson has also been dropped by several charities and media outlets.
ITV sources told The i Paper she will no longer appear on daytime programmes such as This Morning or Loose Women.
A children’s book she wrote, Flora and Fern: Kindness Along The Way, was pulled from publication last month.
Her charity, the SF Trust, has also deleted all references to her former title from its website.
Friends say Ferguson is considering options to restore her finances, including a potential interview with Oprah Winfrey, who has previously hosted her twice – in 1996 after her separation from Andrew and in 2010 after a News of the World undercover sting.
The interview could reportedly fetch up to US$1 million ($1.8m).
Another possible project is a memoir revealing her experience of royal life, which could also attract a six-figure payday. However, friends believe she would avoid saying anything that might harm her daughters’ relationship with senior royals, including the future king Prince William.