The King is taking part in the first Trooping the Colour of his reign to mark his inaugural birthday parade.
He is the first monarch in more than 30 years to take part in the ceremony on horseback.
He will take the royal salute as Colonel in Chief of the Household Division’s seven regiments during the ceremony, staged on Horse Guards Parade, as members of the Royal Family and 8,000 spectators look on.
The colour, or regimental flag, that will be trooped in front of hundreds of Guardsmen and officers will be the King’s Colour of the 1st Battalion Welsh Guards.
The Queen and the Princess of Wales and her children were taken to the ceremony in a carriage with the Duke of Edinburgh, Prince of Wales and Princess Royal also on horseback.
Following the ceremony, the royal family will head back to Buckingham Palace and gather on the balcony to watch an extended military flypast after the display on coronation day had to be scaled down due to bad weather.
Around 70 aircraft from the Royal Navy, British Army and Royal Air Force will take part – including aircraft from the Battle of Britain Memorial Flight, the C-130 Hercules on its final ceremonial flight, modern Typhoon fighter jets with a red, white and blue finale from the Red Arrows.