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.
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.