Prince George duets with Prince William at Party at the palace: WATCH

The royals waved flags and sang along to the anthem

Prince William and Prince George delighted viewers of the Platinum Party in the Palace concert on Saturday night, as they were filmed singing Sweet Caroline together.

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.

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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.

MORE: Prince William makes cheeky comment about the Queen during Jubilee concert

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.

STORY: Princess Charlotte looks so grown up at Party at the Palace

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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