The Sun can reveal the man, 22, was able to scale a fence at the front of the building to get inside the 93-year-old monarch’s home, where he was able to attempt to bang down the doors of the residence.
Sources say it took police on site around four minutes to apprehend him while they tried to assess whether he was armed - causing panic within the Palace about the massive security breach.
There are fears the man was attempting a copycat operation to Michael Fagan who was famously able to enter the Queen’s bedroom almost 37 years ago to the day.
Our royal insider revealed: “There was an intruder at the Palace while the Queen lay sleeping in her bedroom and he was on the loose for a good few minutes before the cops arrested him.
“He was determined to get inside the palace and was banging down the doors but, thank goodness, this time everything was locked up.”
'HOW IS IT POSSIBLE?'
The royal standard flag was flying at the Palace, prompting the suggestion the intruder knew the Queen was at home and may have been trying to access her bedroom.
The royal source added: “This intruder got into the Palace almost 37 years to the day since Michael Fagan ended up getting inside the Queen's bedroom. There is a genuine theory and very real fear that he could have been launching a copycat operation.
“The police eventually detained him under the mental health act, but they have to ask some serious questions about how he got on and how he had so long to roam free.
“How is it possible all these years later that someone can climb the fence and get right inside the grounds without being stopped immediately? What about heat sensors and CCTV? The Queen’s safety should be paramount.”
QUEEN IN AT THE TIME
Buckingham Palace insiders last night revealed the Queen is to be informed of the incident.
A Palace source explained: “Of course this will be a matter of great interest to the Queen. She knows exactly what is going on at the Palace.”
The Sun understands the Queen was the only senior royal sleeping at the Palace because of the refurbishment work going on. We don’t believe the Duke of Edinburgh was there.
Another source added: “Everyone knows the Queen was there because the Royal Standard was flying.
“We are all relieved that the doors were locked this time and he wasn’t able to get inside where the Queen was.”
STRING OF PALACE INVASIONS
The intrusion is a devastating blow to the security operation at the palace, which has been struck by a number of similar incidents.
The most famous was at 7:15 a.m. on the morning of July 9, 1982, when unemployed labourer Fagan scaled the walls and entered the Queen’s bedroom.
As the 31-year-old schizophrenic began to tell his monarch about his family problems, she pushed the panic button by her bed but got no response.
She then telephoned the switchboard but the operator thought it was another member of staff playing a joke and replied: “Yes, dear, you’re the Queen, we’ll send someone up!”
Fagan smashed a large glass ashtray and threatened to slash his wrists as he sat on the end of the Queen’s bed.
It was the Queen’s long-time aide Paul Whybrew – still employed by the Queen to this day – who came to the rescue.
He returned with the corgis and as the Queen called him in he saw Fagan and began talking to him. Paul offered him a drink and then, when his guard was down, manhandled him outside where a maid fetched a policeman.