The guard was participating in the popular Changing of the Guard, but slipped sideways and fell to the ground.
Unfortunately for him, the embarrassing incident was caught on camera and witnessed by hundreds of tourists.
But the guard carried on and pretended nothing had happened.
Tourist David Meadwell, 50, was the man who recorded the fall.
"The first group changed successfully, without any problems, and so I decided to video the next group doing it," Mr Meadwell told The Daily Mail.
"I didn't expect anything like this to happen - he went flying. There were quite a few people around and there were a lot of gasps as he went down.
"I think he just slipped on a manhole cover, he looked so embarrassed".
The YouTube video got 1,399,744 views so far.
The Ministry of Defence refused to comment.
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