PLATINUM JUBILEE: Queen makes surprise appearance at Party at the Palace
The pair appeared in good spirits as they waved Union Jack flags and sang along with Sir Rod Stewart's rendition of the Neil Diamond hit, which was recently voted by BBC Radio 2 listeners as the Thank You Day Jubilee Anthem.
WATCH: Prince George and Prince William sing Sweet Caroline
"Prince George singing along to Sweet Caroline with his dad and waving his flag, a British classic," one viewer tweeted, while another commented that the moment was "Beyond cute."
"Did you see little Prince George singing along to Sweet Caroline with Rod Stewart! Aww," a third asked.
Prince George and Princess Charlotte sat front row in the royal box along with their parents for the party on Saturday night, which featured performances from artists including Queen and Adam Lambert, Sir Elton John and Andrea Bocelli.
Prince George and Prince William waved flags and sang in the royal box
It was the second time in just a few hours that the Cambridges enjoyed a performance of the popular anthem, as earlier in the day Prince William and Kate both tried their hand at conducting an orchestra's rendition of the song during rehearsals for a Platinum Jubilee concert at Cardiff Castle.
Prince George and Princess Charlotte joined their parents as they enjoyed another day celebrating the Queen’s 70-year reign, meeting stars and technical crew at Cardiff Castle, where final preparations were underway for a concert honouring the monarch in the grounds.
The Cambridges sat front row in the royal box for the concert
The royal siblings were making their first official visit to Wales with their parents, but they were not joined by their younger brother Prince Louis, four, who also missed out on the concert as he was likely too young to attend.
Prince George, Princess Charlotte and Prince Louis have played a key role in the Queen's Platinum Jubilee celebrations, and also attended the Trooping of the colour ceremony and made an appearance on the balcony at Buckingham Palace beside their great-grandmother, the Queen, on Thursday.
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