BBC royal correspondent Nicholas Witchell says the prince and his wife flew to Scotland on Sunday, although they did not travel on a scheduled flight.
He adds that they were both tested on Monday and got their results last night.
Prince Charles’s last public engagement was on 12 March – the same day he last saw his mother, the Queen. Palace officials say the Monarch remains in “good health”.
The prince, 71, and his wife, the Duchess of Cornwall, 72, are self-isolating at Birkhall on the Balmoral Estate in Aberdeenshire.
A Palace statement said: “The tests were carried out by the NHS in Aberdeenshire where they met the criteria required for testing.”
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