Charles arrived at the Aberdeenshire summer retreat in Scotland on Tuesday and Prince William and Kate have since been pictured at the estate, as well as the Princess Royal and her brother the Duke of Edinburgh.
William and Kate have not reacted publicly to the news of the Duke and Duchess of Sussex's return but may have private conversations with the wider family during the break.
The future monarch is estranged from his younger brother after Harry levelled a barrage of accusations against him, his wife the Princess of Wales and the King and Queen after stepping back from royal duties.
Some reports had suggested the family was holding a "summit" over the return of the Sussexes.
But it is understood the gathering is in keeping with a longstanding annual tradition of members of the extended royal family staying at Balmoral during the northern hemisphere summer.
Charles, who only found out about Harry and Meghan's plans three days before they became public, is said to be happy to be able to spend more time with the Sussex family but remains adamant they will not be working royals.
Harry and Meghan have not commented on their reasons for relocating to the UK, although a source close to the duchess did not deny reporting that she has been offered an acting role in an upcoming project in the country.
Their move comes after Harry and Meghan took their children to see their grandfather the King for the first time in four years at his Highgrove home in July.
The couple are not thought to have raised the prospect of a return to the UK at that meeting.
Questions have been raised over the Sussexes' finances as decisions about whether taxpayers will foot the bill for their security remain unanswered but the duchess will continue to run her lifestyle brand, As Ever, from their new base.
UK Prime Minister Andy Burnham would not be drawn on Harry and Meghan's security arrangements, saying the funding for it was a "private matter".
After his level of protection changed when he stepped down as a working royal in 2020, Harry has faced a protracted legal battle with the government over security for him and his family while they are in the UK, and their arrangements may change when they become permanent residents again.
At the end of June, Harry was still waiting for a review by the Risk Management Board, part of the process by which the Executive Committee for the Protection of Royalty and Public Figures (Ravec) rules on his security requirements – and he and his security detail were working to try to ensure his July visit could go ahead safely.
Individual decisions on protective security for key public figures, including members of the royal family, are made by Ravec – which includes the Home Office, Metropolitan Police and Royal Household – which advises an independent chairperson.
Rumours have abounded as to where Harry and Meghan will choose to settle, with suggestions including rural locations in the Cotswolds.
Archie and Lili are expected to start at their new schools in early September and it is understood there will be no published photographs of the children when they start or on their birthdays, as for working royals.
UK media organisations have been instructed by the press regulator not to disclose the location of their new family home nor the children's school.
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