The King is to take part in the Trooping the Colour ceremony next month and will inspect the soldiers from a carriage, it is understood.
The King’s Birthday Parade is going ahead on June 15 despite the upcoming general election, the Army confirmed.
Charles, who has been diagnosed with an undisclosed form of cancer, returned to official duties last month.
The celebration traditionally involves the royal family gathering on the balcony of Buckingham Palace after a display of pomp and military pageantry.
It is unclear whether the Princess of Wales, who is undergoing chemotherapy after a cancer diagnosis, will attend.
The annual Trooping the Colour has marked the official birthday of the British sovereign for more than 260 years.
Charles marked his first Trooping as King last year, riding onto Horse Guards in Whitehall as the nation’s head of state six weeks after his coronation.
Almost 1,000 soldiers and around 240 horses from the Household Division are performing a “khaki rehearsal” of the parade in London on Thursday.
The rehearsal is taking place on Horse Guards Parade in front of an audience of more than 1,000 people as a test for soldiers and horses who have yet to rehearse publicly.
Lieutenant Colonel James Shaw, the Brigade Major of the Household Division, will confirm the troops are of the required standard before they perform at The Major General’s Review on Saturday, The Colonel’s Review on June 8 and Trooping the Colour on June 15.
The Army said the Number 9 Company, Irish Guards, has been chosen to troop their colour, which means the regiment will march its ceremonial flag through the ranks at the King’s official birthday event.
The Band of the Irish Guards with the Pipes and Drums will march the troops onto the parade ground, led by regimental mascot Turlough Mor, an Irish Wolfhound also known as Seamus.
The music is set to have a “distinctly Irish theme”, including Celtic folk tunes and new music composed for the parade.
The royal family has postponed engagements “which may appear to divert attention or distract from the election campaign” after Rishi Sunak last week called a surprise summer General Election for July 4, but have continued with garden parties at Buckingham Palace and the King and Queen’s D-Day 80th anniversary appearances in Portsmouth and Normandy in June are expected to go ahead as scheduled.
