Five Guardsmen were stretchered off Horse Guards Parade after fainting in the sweltering heat during the Trooping the Colour ceremony.
Dressed in his full uniform, including a bearskin, one soldier fell forward out of his formation as temperatures soared to 25C in nearby St James's Park.
As the band marched towards his position, the guardsman in the second row collapsed to his knees before landing face down with his hands to his side.
His colleagues maintained position as the ceremony to mark the Queen's birthday continued around him.
Three soldiers then walked to his aid.
One removed his bearskin before the guardsman was lifted on to a stretcher one minute and 17 seconds after he fainted.
At least four other guardsmen at the event are believed to have fallen victims to the higher-than-average temperatures on Saturday, also fainting.
An Army spokeswoman said: "We can confirm that during the Queen's birthday parade today a small number of soldiers fainted.
"It is an extremely hot day and all were removed from the Parade and checked by medical staff where they were hydrated."