The warning came during a private confrontation when Charles was still the Prince of Wales.
The Duchy, a vast estate providing income to the Prince of Wales, is now under the ownership of Prince William, following Charles' ascension to the throne.
A Channel 4 Dispatches documentary and The Sunday Times investigation revealed that King Charles' estates received millions in funds from public bodies and charities.
According to royal author Tim Shipman, Johnson issued a stern warning to the Prince: "I wouldn't talk about slavery if I were you, or you'll end up having to sell the Duchy of Cornwall to pay reparations."
The warning came during the Commonwealth Heads of Government Meeting in Kigali, where Charles expressed deep personal sorrow over the legacy of slavery.
During the meeting, King Charles, then Prince, spoke of his own understanding of slavery’s enduring impact, stating: "I cannot describe the depth of my personal sorrow at the suffering of so many.
Following King Charles' ascension to the throne, the Duchy of Cornwall passed into the hands of Prince William, but the estate continues to be a source of both wealth and controversy.
The documentary revealed that the royal estates had received nearly £50 million from contracts with public bodies like the NHS and the British Army.
In his first speech as Head of the Commonwealth, King Charles called for learning from the past: "None of us can change the past, but we can commit with all our hearts to learn its lessons."
Boris Johnson accused Charles of being captured by "woke ideology" after the Prince expressed concern over the British colonial legacy.
With Prince William now in control, the Duchy of Cornwall remains a crucial part of the royal family’s wealth.
However, questions about its financial dealings and the ongoing impact of Britain’s colonial history continue to dominate the debate.