Simon Case, Prince William's ex-aide, played a crucial part in Meghan Markle and Prince Harry's departure from the Royal Family.
According to court documents, Case was responsible for drafting a significant document outlining the Duke and Duchess of Sussex's decision to step away from royal duties.
This week, Prince Harry faced defeat in a legal battle against the Home Office over his security arrangements in the UK.
Details regarding Harry and Meghan's transition away from their royal roles in 2020 were disclosed in a 51-page judgment from the court case.
Case authored a paper on behalf of the Royal Family which set out the steps for the Duke and Duchess of Sussex to quit royal duties, The Times reports.
Court papers state: "On January 8, 2020, an announcement was made in relation to the claimant [the Duke of Sussex] stepping back from official royal duties and a public role.
"On January 11, 2020, Sir Edward Young [the late Queen’s private secretary] emailed the claimant with a draft paper, which was largely the work of Simon Case, concerning the detailed arrangements to give effect to the announcement.
"Following a meeting at Sandringham on January 13, 2020, what the claimant describes as ‘an agreement of sorts’ was reached, which has been described in the media as the ‘Sandringham Agreement’."
It comes as Prince Harry has vowed to launch an appeal against the High Court's ruling on his security battle with the Home Office.
The Duke of Sussex suffered a crushing defeat as the judge ruled in favour of the 2020 decision made by the Executive Committee for the Protection of Royalty and Public Figures (Ravec).
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