Charles instructed the Lord Chamberlain to contact government departments and the military following requests for guidance after Harry and Meghan in August.
The letter makes clear the duke and duchess do not use their HRH styles and any operational security issues involving them should be directed to the relevant police authorities.
The clarification came on the same day Harry visited Tower Bridge Studios - the first time he has been pictured since relocating to the UK.
The London filming and podcast studio posted on Instagram about the royal visit.
"Thank you Prince Harry for choosing Tower Bridge Studios! It was an absolute pleasure to have you in our studios and signing our wall - we look forward to welcoming your back again."
The visit is likely to fuel speculation Harry could have recorded an episode for a podcast or footage for one of his charity ventures.
The Sussexes stepped down as working royals in 2020 but returned to the UK on August 26 with rumours rife as to the reason.
In the run up to their return, the King was said to be glad to be able to spend more time with the family, but remained adamant they would not be working royals and there was no half-in, half-out option.
"Following the decision by the Duke and Duchess of Sussex to relocate to the UK, we have received a number of requests for guidance. To help avoid doubt or confusion, The King has directed that the following information be shared," the letter said.
"It is well known that in January 2020 the Duke and Duchess stepped down from undertaking representative duties on behalf of The Sovereign, and are no longer working Members of The Royal Family.
"This position, distinct from the State and Royal duties undertaken by the working Royal Family, and with the personal latitude it brings the couple in respect of financial independence and protection of their privacy as they would wish, will continue to be fully respected."
Harry is expected to carry out his annual series of engagements around the WellChild awards, normally held in mid-September, and it has been reported Meghan is aiming to rekindle her work with her UK-based charities.
The couple's children Prince Archie and Princess Lilibet are expected to be starting school in the UK near the family's reported countryside retreat.
The letter stipulated "there is no change to the current status of the Duke and Duchess of Sussex".
"Their styles as His and Her Royal Highness remain in abeyance and are not used.
"The charitable work of the duke and duchess is a personal matter for them both and undertaken in their private capacity.
"In short, their position is akin to private citizens with commercial and charitable interests."
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