The King's guards at Buckingham Palace perform grand rendition of 'Happy Birthday' for King Charles

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

Performing the Changing of the Guard ceremony on Monday, the Household Cavalry Regiments 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 King Charles," added another.

Charles, who became the oldest monarch in British history when he succeeded his mother on the throne in September, will enjoy his first birthday as king in private with no public engagements planned.

Happy 74th birthday King Charles!