The suspect had screamed at cops for five minutes and walked towards the Palace gates.
Warehouse supervisor Gareth Scanlan, of Rugby, Warwicks, who was at the scene, told The Sun on Sunday: “He was trying to get closer or inside.
“There was a lot of shouting before the guard went in with his bayonet — it was shocking.
Sightseer Gareth, 32, added: “The man said, ‘Oh, you’re a big boy now,’ and the guard — who I think was Scottish judging by his accent — said, ‘Yes, I am a big boy.
“Then the guard actually pushed the guy away.”
Police ushered the man away and gave him “words of advice”. No arrests were made.
Army chiefs are believed to have backed the soldier, from the Coldstream Guards, though they must not normally leave their posts unless a royal is under threat.
SAS hero Andy McNab said: “He should get a promotion for having the balls to go and stop something. He was on ceremonial duties, but he is still there to protect the Palace.
“If it happened at the White House, someone would have got shot. Yet here we think it’s OK to have a pop at the people protecting our country.”
Colonel Richard Kemp, ex-Commander of British Forces in Afghanistan, said: “The soldier was absolutely right to get involved if he felt there was a risk of the situation escalating.”
The drama happened near the Palace’s North Centre Gate on Friday.
An Army spokesman said: “We are very clear that the Metropolitan Police lead on royal security arrangements including outside the Palace itself.”
QUEEN’S Guards are fully operational soldiers who have often completed tours of Afghanistan.
They are on sentry duty for two hours at a stretch and can stamp and shout if visitors try to hinder them.
If a threat continues, they are allowed to point their weapons.