The guard was standing at the foot of the late monarch’s casket when he suddenly fell to the floor.
Footage taken from a live broadcast which was posted to social media around 1am captured the collapse.
The black-clad guard was holding a ceremonial staff when he appeared to faint, with nearby officials quickly rushing to his aid.
As he was tended to, the live vision quickly faded to exterior, night-time vision of the building. Over an hour later, the broadcast had still not returned to the scene inside the hall.
Soldiers in ceremonial uniform are maintaining a constant, 24-hour vigil around the Queen’s coffin while she is lying in state.
The guards from units which include the Sovereign’s Bodyguard, the Household Division or Yeoman Warders of the Tower of London are required to remain completely still at the four corners of the catafalque.
While the soldiers rotate every 20 minutes, the hours of remaining completely still while standing are six-hours in length.
The incident happened on the first night of the Queen’s lying in state which gives members of the public the chance to pay their respects to the late monarch.