Prince William's former aide, Simon Case, played a pivotal role in Meghan Markle and Prince Harry's Royal Family exit.
Case drafted a key document on the Duke and Duchess of Sussex quitting royal duties, court papers show.
This week, Prince Harry lost a High Court battle against the Home Office regarding his security in the UK.
The 51-page judgement in the case revealed details about Harry and Meghan stepping down as working royals in 2020.
Prince William's former aide, Simon Case, played a pivotal role in Meghan Markle and Prince Harry's Royal Family exit.
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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’."
Meghan Markle and Prince Harry stepped down as working royals in 2020
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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).
Harry was fighting Ravec's 2020 decision that he should receive a different degree of protection when in the UK.
The court found there had not been any unlawfulness in reaching the decision of February 28 2020 and that any departure from policy was justified.
Prince Harry has vowed to launch an appeal against the High Court's ruling on his security battle with the Home Office.
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The Duke of Sussex had challenged the Home Office over his right to automatic police protection in the UK.
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The court has also found that there has been no unlawfulness on the part of Ravec in respect of its arrangements for Prince Harry's visits to the UK following the 2020 decision.
The Duke of Sussex had challenged the Home Office over his right to automatic police protection in the UK after he was stripped of taxpayer-funded security.
Harry had been told that he would no longer receive the “same degree” of protection when in the UK from 2020.
The decision means the level of Prince Harry's taxpayer-funded personal security will not change when he travels to the country in the future.