While this year's Trooping the Colour will be slightly different due to the King's ongoing cancer treatment, the monarch will no doubt still mark the event in his military uniform.
The King often wears the colours of the Royal Air Force, having served with the aerial force in 1971 and becoming a fully qualified helicopter pilot in 1974. The King served in the military during the 1970s, retiring in 1976, however, he has kept a close relationship with the RAF and holds the highest titles in the regiment.
In 2012, his mother bestowed upon him the highest possible rank for all three forces, making him a Field Marshal, an Admiral of the Fleet and a Marshal of the Royal Air Force.
Although the King never saw active military service, unlike his younger brother Prince Andrew and son Prince Harry, he still wears medals, although these reflect his lack of combat experience.
The King's medals
The King has 11 medals on his uniform and these are: the Queen's Service Medal, the Coronation Medal, the Silver Jubilee Medal, the Golden Jubilee Medal, the Diamond Jubilee Medal, the Platinum Jubilee Medal, the Naval Long Service Good Conduct Medal, the Canadian Forces Decoration, the New Zealand Commemorative Medal, the New Zealand Armed Forces Award and the Garter Star.
The Queen's Service Medal, now known as the King's Service Medal, is handed out for either community service or for public services. Meanwhile, the Coronation Medal is handed out for the coronation of a new monarch, with the Jubilee Medals coming for the relevant Jubilee celebrations.
The Naval Long Service Good Conduct is awarded to those who serve in the Royal Navy for at least ten years. The Canadian Forces Decoration is similarly handed out for long service, typically being awarded for 12 years worth of service.
The King's New Zealand Commemorative Medal marks the 150th anniversary of the signing of the Treaty of Waitangi, and the monarch's siblings were also recipients of the medal, while the New Zealand Armed Forces Award is also awarded for long service.
Charles will no doubt be proud of his Garter Star, which marks his place in the Order of the Garter, the most senior order of knighthood in the United Kingdom, which dates back to 1348.
King's military service
Like many senior members of the royal family, Charles completed military service during his youth. The then Prince of Wales completed his Royal Air Force training during his second year at Cambridge, being presented with his RAF wings in 1971.
He also trained with the Royal Navy, serving on the HMS Norfolk, HMS Minerva, HMS Jupiter and the HMS Hermes. His final ten months of service saw him as a commander on the HMS Bronington, a coastal minehunter.
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Through his military service, Charles is also a fully qualified helicopter pilot, and also trained with the Parachute Regiment in 1977.