The protest is the latest in a catalogue of royal security breaches

Former royal press officer Dickie Arbiter said security officials needed "to go back to the drawing board".

The Shadow Home Secretary, David Davis MP, said lessons from previous incidents "had not been learnt".

And Mark Oaten, Lib-Dem home affairs spokesman, called the breach "another embarrassing breakdown".

Mr Davis added: "It beggars belief that we have had four major breaches of royal security in a year. When the threat from terrorism is at an all time high it is worrying that episodes like this can still take place."

"There must be serious questions asked about the ability of our authorities to protect the Royal Family," Mr Oaten said.

A Home Office spokeswoman declined to comment on the incident and apparent security lapse, referring all inquiries to the Metropolitan Police.

This will go round the world and everyone will be laughing at us James Whittaker, former Daily Mirror royal correspondent

John O' Connor, a former Scotland Yard Flying Squad commander, said he was "astonished" anybody was able to get through the security cordon considering increased security awareness over the terror threat.

"Clearly there was no-one there with the ability to stop intruders. It's bizarre," he said.

"They are not looking at it from a professional security point of view. They are still looking at what looks good in the public eye. I think it's an absolute disgrace."

Former palace press spokesman Dickie Arbiter said the breach was "probably very easy as the palace is open to the public".

He continued: "If you have a few thousand people around the ambassadors' entrance going to visit the state rooms it probably isn't that difficult for someone to get in quite legitimately then surreptitiously disappear.

"The problem is that the police really have no jurisdiction inside the building. There are wardens on duty throughout the summer, but they cannot possibly know everyone who works there."

Holy of holies

James Whittaker, who was royal correspondent with the Daily Mirror, said it was extraordinary nothing had been learned from past breaches of security.

He added: "It is absolutely incredible that somebody has managed to do this. This is the balcony where a member of the family stands on the great day when they get married. It is a bit of a holy of holies.

"The queen at Balmoral will be absolutely livid. She doesn't laugh at things like this. She doesn't find them even vaguely amusing. She just finds it thoroughly inept that they can't keep the people out. I think there will be absolute silent fury.

"And I think that the Home Secretary is going to have to explain. In the old days, a decent gentleman would have resigned. I think it is an incredibly serious matter. This will go round the world and everyone will be laughing at us."