Five guardsmen fainted amid sweltering temperatures during the Trooping the Colour ceremony in central London.
Dressed in full uniform, which includes a bearskin, they had to be stretchered off Horse Guards Parade.
As temperatures reached 25C (77F), one soldier fell forward onto his knees, out of formation, before landing face down with his hands by his side.
His colleagues initially maintained their position before three soldiers came to his assistance.
At least four other guardsmen are believed to have fainted too as the mercury soared.
“We can confirm that during the Queen’s birthday parade today a small number of soldiers fainted,” an Army spokeswoman said.
“It is an extremely hot day and all were removed from the Parade and checked by medical staff where they were hydrated.”
The Queen, who travelled in a carriage with the Duke of Edinburgh, has hailed the UK for being “resolute in the face of adversity” after three terror attacks and the Grenfell Tower fire.
In a statement released on her official 91st birthday, she said that while the day was traditionally a celebration, “it is difficult to escape a very sombre national mood”.
During the ceremony, 1,600 soldiers and 244 horses from the Household Division were on parade.
Later, the royal family watched the traditional RAF fly-past from the Buckingham Palace balcony.