The Duchess of York was diagnosed with malignant melanoma after a cancerous mole was removed during her breast cancer treatment.
Sarah Ferguson has had several moles removed and analyzed during reconstructive surgery after a mastectomy, her spokesperson said.
The Duchess "remains in good spirits" although it is "worrying" to be given another cancer diagnosis.
She is the third royal to announce a medical procedure this week.
Her spokesperson told the BBC: "Her dermatologist asked for several moles to be removed and analyzed at the same time as the Duchess was undergoing reconstructive surgery following her mastectomy, and one of them was identified as cancerous.
"She is undergoing further tests to make sure it was caught at an early stage. Obviously, another diagnosis so soon after breast cancer treatment is worrying, but the Duchess remains in good spirits. The Duchess would like to thank the entire medical team that supports her, especially her dermatologist, whose vigilance ensures that the disease is detected when it should be. She believes that her experience highlights the importance of checking the size, shape, color, and structure and the appearance of new moles, which can be a sign of melanoma".
The statement also thanked the private medical team that supported the royal during her treatment.
The Duchess confirmed her breast cancer diagnosis in June last year, following a routine mammogram.
She then had a single mastectomy at King Edward VII Hospital in London, a private clinic used by senior royals.
The Duchess, 64, was married to the Duke of York, Prince Andrew, for 10 years before they divorced in 1996.
The two continue to live in a shared home at Royal Lodge, a Crown Estate property in Windsor Park.
They have two daughters - Princess Beatrice, 35 years old, and Princess Eugenie, 33 years old, as well as three grandchildren./BGNES
