On the west coast of the United States, where Harry lives with his pregnant wife and their son, it was around 3am.
Palace aides contacted the US embassy in London which called the Santa Barbara sheriff’s office, according to the Daily Telegraph.
It is understood an officer was dispatched to the the Duke of Sussex’s home in Montecito, around 10 miles away.
While it is not clear if Royal Family members attempted to contact him before American police, Buckingham Palace aides would have been aware of the need to alert the duke before the media.
Prince Harry, 36, was reportedly asleep at the time and had not been able to be contacted any other way.
Police passed on the news of Prince Philip’s death to a security guard and the Duke of Sussex was then made aware.
A spokesperson for Santa Barbara sheriff’s office told the Daily Telegraph: “We did go to the house on the 9th at 3am on the request of Buckingham Palace.
“The call note says it was a call for service from the US embassy in London.”
They confirmed the purpose of the visit was to inform Prince Harry that his grandfather had died.
The death was announced by Buckingham Palace around one hour later, at midday UK time.
Prince Harry issued a personal tribute days later, describing his grandfather as “master of the barbecue, legend of banter, and cheeky right 'til the end”.
He flew back to the UK for the funeral, where he self-isolated at Frogmore Cottage, the Windsor home the Sussexes have kept as a UK base, ahead of the ceremony.