The Queen's guards at Buckingham Palace perform sweet rendition of 'Happy Birthday' for Prince George

They're notorious for being extremely serious, but it appears even the Buckingham Palace guards couldn't resist paying tribute to Prince George on his ninth birthday.

Performing the Changing of the Guard ceremony on Friday, the Band of the Irish Guards treated onlookers to a rendition of the iconic song, 'Happy Birthday'.

As the guards usually stick to ceremonial music, viewers were left stunned with many posting on social media about the moment.

"What a wonderful gesture. Proud to be British," one Instagram user wrote.

"Amazing gift to Prince George! Our future King," added another.

Prince George Alexander Louis of Cambridge was born in the private Lindo Wing of St Mary’s Hospital in London in July 2013 and made his first public appearance a day later, when his parents stood on the hospital steps in front of a huge crowd of photographers and camera crews.

Happy birthday Prince George!