Prince William is likely to attend his uncle's funeral but Prince Harry is expected to miss the event.
Lord Fellowes, who died last month, was married to Lady Jane Spencer, the sister of Diana, Princess of Wales.
He also served as private secretary to Queen Elizabeth II for nine years.
Prince Harry remains close to his aunt Lady Jane and last saw her in May 2024 during his most recent trip to the UK.
Prince William is likely to attend his uncle's funeral but Prince Harry is expected to miss the event.
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However, the Duke of Sussex feels it is too dangerous to travel to the UK with the current level of security that he receives in the country of his birth.
Prince William and Prince Harry are not on speaking terms and have not seen each other since King Charles's Coronation in May 2023.
The Duke of Sussex chose to miss the Duke of Westminster's wedding in June 2024 to avoid an awkward meeting with his brother.
The Prince of Wales was an usher at the wedding and has a close bond with the Duke of Westminster.
Prince Harry is also close to Hugh Grosvenor but declined an invitation to the wedding.
The Duke of Sussex chose to miss the Duke of Westminster's wedding in June 2024 to avoid an awkward meeting with his brother.
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It comes as Kate and William have issued a Prince Harry and Meghan Markle "ban" as the Princess of Wales continues to focus on her health, according to a source.
Since stepping down as working royals in 2020, the Sussexes have each taken public swipes at Princess Kate over the last few years.
The most recent digs came in the Duke of Sussex's autobiography Spare, which was published in January 2023.
The Princess of Wales was diagnosed with cancer earlier this year after medical tests found the disease was present following significant abdominal surgery at the London Clinic hospital.
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The Princess of Wales was diagnosed with cancer earlier this year after medical tests found the disease was present following significant abdominal surgery at the London Clinic hospital.
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Princess Kate will travel with Prince William, Prince George, Princess Charlotte and Prince Louis to Balmoral this summer.
Members of the monarchy have an annual summer break at the Scottish estate in line with the Royal family’s custom.
King Charles has continued the tradition with the Waleses, Edinburghs and Yorks all expected to attend this summer but the Sussexes have not been invited.
The source said: "Kate and William have put a self-imposed ban on anything Meghan and Harry-related, as they take time out to enjoy some much-needed respite and family time this summer.
"Kate’s still very much focussing on her healing, and they decided to create a firm boundary to protect her from all the stress and upset Harry and Meghan create."