The Queen looked festive in a red hat and coat by her personal dresser Angela Kelly and accessorised with a ruby and diamond spray brooch.
Meanwhile, Kate, who earlier in the day was seen dressed in a red Alexander McQueen coat and Emilia Wickstead tartan skirt, changed into a dark green Catherine Walker coat with a faux-fur collar. The duchess also wore the Queen's diamond pendant earrings, which she has borrowed on previous occasions.
During the socially distanced engagement, the royals listened to a selection of Christmas carols performed by The Salvation Army Band and the Queen could be seen quietly singing along.
The monarch chatted with her youngest son Prince Edward and his wife Sophie, as well as the Cambridges, on the steps in the quadrangle of the Castle.
After the carols, the monarch turned to walk back inside as William said “Bye gran”.
The rest of the royal family stayed to thank local volunteers for their efforts during the coronavirus pandemic.
Notably absent was the Duke of Edinburgh, despite the event happening on the doorstep of Windsor Castle, where he and the monarch are currently living. While Prince Philip officially retired from royal duties in August 2017, the 99-year-old is often seen accompanying the Queen.
William and Kate’s UK-wide tour saw the couple travel to cities and towns on the royal train to thank key workers, volunteers and communities for their efforts during the pandemic.
The trip provoked some criticisms from Welsh and Scottish ministers, who raised suggestions about the timing of the visits while coronavirus cases were still prevalent.
However, prime minister Boris Johnson said he believes the tour was a “welcome morale boost”.
“The prime minister is delighted to see the warm reception the Duke and Duchess of Cambridge have received on their hugely valuable train tour of England, Scotland and Wales,” a Downing Street spokesperson said.
“The tour will be a welcome morale boost to frontline workers who have done so much during the pandemic.”
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