Ngozi Fulani, who was born in Britain and works for a domestic abuse support group, wrote on Twitter that the royal aide had repeatedly asked her: "What part of Africa are you from?" when she attended an event hosted by King Charles's wife Camilla, the queen consort, on Tuesday.
"We take this incident extremely seriously and have investigated immediately to establish the full details. In this instance, unacceptable and deeply regrettable comments have been made," a Buckingham Palace spokesperson said in a statement.
The spokesperson said the individual concerned, referred to by Fulani as Lady SH, wanted to apologise for the hurt caused and had stepped aside from her honorary role with immediate effect.
Neither Fulani nor the palace identified the aide in question, with the spokesperson saying the individual would not be offering any comment. British media said it was Lady Susan Hussey, the 83-year-old godmother of Charles's son and heir Prince William.
William's spokesperson said he was really disappointed to hear about the incident.
"Obviously, I wasn’t there, but racism has no place in our society," the spokesperson said. "The comments were unacceptable, and it is right that the individual has stepped aside with immediate effect."
The incident is the latest to embroil the royal family in allegations of racism, after Charles's younger son Prince Harry and his wife Meghan made accusations in an interview with Oprah Winfrey in March 2021.
Meghan, in the interview, said one unnamed member of the family had asked, before their son Archie was born, how dark his skin might be.
The allegation clearly stung the monarchy, which promised any such issues would be treated very seriously, and prompted William, Harry's older brother to remark days later: "We're very much not a racist family."
The exchange with Fulani had occurred at a 'Violence Against Women And Girls’ reception at the palace, where guests included Ukraine's first lady Olena Zelenska, Belgium's Queen Mathilde and Jordan's Queen Rania.
In her account posted on Twitter, Fulani, who works for Sistah Space - a group which provides support for women of African and Caribbean heritage who have been affected by abuse - said about 10 minutes after she arrived, the aide approached her and moved her hair to view her name badge.
After being asked a number of times what part of Africa she was from, Fulani said she replied: "I am born here and am British."
The aide responded: "No, but where do you really come from, where do your people come from?"
Last year, a senior royal source said Buckingham Palace had not done enough on diversity, although it has been endeavouring to boost the number of staff from ethnic minorities.
"We have reached out to Ngozi Fulani on this matter, and are inviting her to discuss all elements of her experience in person if she wishes," the palace spokesperson said.
"All members of the Household are being reminded of the diversity and inclusivity policies which they are required to uphold at all times."