Protesters thinking of imitating a fathers' rights campaigner who climbed onto a ledge of Buckingham Palace risk being shot, the Met police chief has warned.
The protest raised questions about security in royal palaces
Sir John Stevens said copycat protesters should "think twice" before going ahead with similar stunts.
He made his comments after examining a report into Monday's protest by a Fathers 4 Justice campaigner.
He also suggested the creation of an offence of trespass on royal property.
Jason Hatch's protest sparked queries over royal security.
The 32-year-old, dressed in a Batman costume, spent five hours outside the palace and was eventually removed by police on a cherry-picker crane.
He and a fellow campaigner - David Pyke, 48 - were arrested and have been released on police bail.
In a statement, Sir John, who is the Metropolitan Police Commissioner, said: "Having now examined and considered the report in detail and listened to the officers involved I wish to make the following clear: if anyone was encouraged by yesterday's Batman stunt - think twice before you go ahead - you risk being shot.
"At this current level of terrorist threat it is unacceptable for our officers to be placed in the difficult and unnecessary position of having to decide in a split second 'protestor or terrorist?'"
He said Met officers had showed their "professionalism and good judgment" but warned: "I would not want anyone to be injured or killed in the future because of someone taking a chance too far."
Change in law
He also said there needed to be a change in the law so that it was illegal to trespass on royal premises, ensuring perpetrators could be brought before the courts.
The Armstrong report, which looked at a breach in security at Windsor Castle during Prince William's 21st birthday party, recommended a change in the law.
Sir John said he was confident Home Secretary David Blunkett would introduce it as swiftly as possible.
Click here to see how the intruder reached the balcony
On Tuesday three police officers were seen standing at the spot where a ladder was used for Mr Hatch's ascent. His fellow campaigner Mr Pyke, who dressed as Batman's sidekick Robin, was stopped by armed police at the scene.
Mr Hatch, from Cheltenham in Gloucestershire, was arrested on suspicion of criminal damage.
Mr Pyke, 48, also from Cheltenham, was questioned about aiding and abetting a criminal act.
Following the incident Mr Blunkett argued the Met Police had "acted correctly" in assessing the threat posed by Mr Hatch.
Later he insisted that he was "calm but not complacent" about the intrusion and said there was no reason to demand resignations "for the sake of a 24-hour headline".
HOW INTRUDER REACHED ROYAL BALCONY
1 Protesters create a diversion at the front gates of the palace.
2 "Batman" and "Robin" climb over low fence at side of palace grounds while security staff are distracted.
3 "Batman" climbs up a ladder on to the roof of an adjoining building. "Robin" comes down after police threaten to shoot him.
4 "Batman" shuffles from roof along ledge to the edge of the royal balcony, where he stages his high-profile protest.
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