This latest update comes just weeks after it was revealed Prince Andrew was "terrified" about what could happen to his loyal sidekick.
When she first made her secondary cancer diagnosis public, Fergie shared it was a "shock" via her Instagram account, especially considering she had only just overcome breast cancer just months prior.
According to our source, "learning of Sarah's second brush with cancer" left Prince Andrew, 63, shaken to the core - particuarly since she's one of his few remaining royal supporters outside of their daughters, Princess Eugenie and Princess Beatrice.
"Andrew realised how heavily he relies on his ex-wife for emotional support. He's terrified at the thought of anything happening to her," the source says.
Our insider adds that Fergie's now also considering "taking an extended break" to recuperate with her Australian-based sister Jane Ferguson - which is also worrying Andrew.
"He dreads the thought of her being so far away," says our source.
Shortly after her skin cancer diagnosis became public knowledge, Sarah took to her Instagram to share that she was in "good spirits."
"I have been taking some time to myself as I have been diagnosed with malignant melanoma, a form of skin cancer, my second cancer diagnosis within a year after I was diagnosed with breast cancer this summer and underwent a mastectomy and reconstructive surgery," she posted to Instagram.
“It was thanks to the great vigilance of my dermatologist that the melanoma was detected when it was."
Her diagnosis was first confirmed in a statement by her spokesperson to The Telegraph UK.
“Following her diagnosis with an early form of breast cancer this summer, Sarah, Duchess of York has now been diagnosed with malignant melanoma,” the statement read.
"Her dermatologist asked that several moles be removed and analysed at the same time as the duchess was undergoing reconstructive surgery following her mastectomy, and one of these has been identified as cancerous."
The spokesperson also revealed that Sarah was "undergoing further investigations to ensure that this has been caught in the early stages."
"The Duchess wants to thank the entire medical team that has supported her, particularly her dermatologist whose vigilance ensured the illness was detected when it was.
"She believes her experience underlines the importance of checking the size, shape, color and texture and emergence of new moles that can be a sign of melanoma."
In June 2023, the mother of two was diagnosed with breast cancer after a tumour was discovered during a routine mammogram.
She then underwent a single, successful mastectomy.
In November 2023, Sarah made the harrowing revelation that she had almost missed the [mammogram] screening appointment that saved her life.
"I couldn't face a journey into London on a hot day this summer and it was only my sister Jane's insistence that I went, that persuaded [me]." she shared on a long-running British talk show, Loose Women.
She then added that her cancer was completely "symptom-free" and that without the mammogram, she never would have known she had cancer as she "never found a lump" and did not feel ill.