Private secretaries, the finance office, the communications team and other loyal household employees were among those who received the letter, while the thanksgiving service for the Queen was taking place in St Giles’ Cathedral in Edinburgh on Monday.
Public and Commercial Services Union (PCS) condemned the decision to announce redundancies during the period of mourning as “nothing short of heartless”.
Charles’s office will move from Clarence House to nearby monarchy HQ, Buckingham Palace, following the death of the Queen.
The King’s top aide, his principal private secretary Sir Clive Alderton said, in a letter seen by The Guardian, that the change in role for Charles and Camilla, now Queen Consort, meant that Clarence House will be “closed down”.
He said the former Prince of Wales’s personal interests and former activities will no longer be carried out.
The Guardian reported that staff were left livid and shaken by the announcement.
A Clarence House spokesman said: “Following last week’s Accession, the operations of the Household of the former Prince of Wales and Duchess of Cornwall have ceased and, as required by law, a consultation process has begun.
“Our staff have given long and loyal service and, while some redundancies will be unavoidable, we are working urgently to identify alternative roles for the greatest possible number of staff.”
Any staff being made redundant will be offered enhanced redundancy payments, and no staff will be affected for at least three months.
The Households of the Queen Mother and the Duke of Edinburgh were closed after their deaths.
It is understood legal advice taken by the household required the information to be shared with staff at the earliest opportunity, despite efforts to delay until after the Queen’s funeral.
PCS general secretary Mark Serwotka said: “The decision of Clarence House to announce redundancies during a period of mourning is nothing short of heartless.
“The self-same staff have been working tirelessly over this period in supporting the activities of the new King and ensuring continuity across the estates of the Prince of Wales.
“While some changes across the households were to be expected, as roles across the royal family change, the scale and speed at which this has been announced is callous in the extreme.
“Least of all because we do not know what staffing the incoming Prince of Wales and his family might need.
“PCS will continue to work with the Royal Households to ensure that those staff have full job security.
“We remain committed to supporting those colleagues across the wider royal family’s estates whose futures are thrown into turmoil by this announcement, at this already difficult time.”