Donatus and Philipp were seen arriving at Windsor together in a vehicle, both wearing black face masks and suits, as were all the other royals.
When the Queen dies, he will be transferred to lie alongside her in the King George VI memorial chapel, which houses the remains of her father, George VI, her mother, and the ashes of her younger sister, Princess Margaret.
Royal officials say the coffin is draped in Philip's personal standard, and topped with his Royal Navy cap and sword and a wreath of flowers.
Mourners have been instructed to wear masks and observe distancing inside the chapel and not to join in when a four-person choir sings hymns.
Former Bishop of London Richard Chartres, who knew Philip well, said the prince was a man of faith, but liked things kept succinct.
Philip was the guiding force behind his funeral arrangements and, reflecting his life-long association with the Royal Navy, Buglers of the Royal Marines will sound Action Stations during the service at the duke's request.
Philip spent nearly 14 years in the Royal Navy and saw action in the Mediterranean, Indian Ocean and Pacific during World War II. The ceremony will then conclude with the national anthem.
Queen Elizabeth has not yet addressed Prince Philip's death with the public (aside from a statement released on her behalf announcing his death on April 9), but many family members have shared statements praising Philip's dedication to the monarch, the country and the Commonwealth.
And last week, the day after Prince Philip's death, the Queen's daughter-in-law Sophie, Countess of Wessex tearfully told people gathered outside Windsor Castle that "the Queen has been fantastic".
He played an active role in the royal family before he retired from public engagements in 2017.
The Queen will lead mourners, as they pay their respects to the man she once called her "strength and stay", and whose death closes a remarkable chapter for Britain´s most famous family.
Speaking of his grandfather after returning to the United Kingdom last week, Prince Harry said he was "a man of service, honor and great humor. with a seriously sharp wit".
Television presenter Jeremy Clarkson has warned that Prince Harry and Meghan Markle's marriage may be doomed, as the couple gets ready to welcome their second child.
A British Army Land Rover ambulance is driven away from Windsor Castle in Windsor, west of London, on April 15, 2021, after a rehearsal for the funeral of Britain'sPrince Philip, Duke of Edinburgh.