The Queen’s safety is “under threat” after a crossbow-wielding intruder allegedly broke into the grounds of Windsor Castle on Christmas Day, ex-royal bodyguards said.
Security officers reportedly spotted him scaling the perimeter fence before armed police were sent to the scene shortly after 8.30 am.
A man, 19, who was held within the grounds, has been detained under the Mental Health Act.
Ken Wharfe, the ex-bodyguard to Princess Diana, called for a major security review at Windsor following a third breach this year.
He said: “The Queen’s safety is clearly under threat.
“Fortunately, this individual was caught in time but any system could always be beefed up as any other scenario could be tragic.”
Officers from Thames Valley Police and the Met were alerted while the Queen was preparing to welcome her family at her residence in Berkshire.
The intruder from Southampton was arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon.
Crossbows are often referred to as “lethal but legal” because they require no licence or registration and can be bought on the internet.
A Palace source said: “Fortunately the intruder was apprehended and police were on the scene almost immediately but there is a chill in the air over what could have been.”
Buckingham Palace has declined to comment on the incident.
The Metropolitan Police force confirmed it had taken over responsibility for the investigation.
Dai Davies, the former chief superintendent and head of the royal protection unit, said: “The officers on the scene did their job and averted a disaster and that is first and foremost the most important thing. But once the threat was contained and the individual was in custody, questions will be asked immediately.
“It does seem that Windsor is having an issue with intruders of late and those in charge need to get to the bottom of it straight away before something more sinister comes to pass.”
It is at least the third major security breach at Windsor in the past year.
Five intruders have made it on to royal grounds – two incursions coming just days apart, in April.
The trespassers included a woman who was allowed into Prince Andrew’s Royal Lodge after claiming she was his fiancée.
It was claimed that the 43-year-old woman arrived in Windsor in a taxi and had even persuaded the duke’s security guards to pay her fare.
She spent 20 minutes in the grounds of Royal Lodge after telling security guards she was having lunch with the Queen’s son.
The intruder later made her way into the home and spent 20 minutes wandering around before being held after asking a member of staff where she could find the duke, and saying she was his fiancee and had come to marry him.
Police said the woman was stopped and later sectioned under Section 136 of the Mental Health Act but there was no risk to anyone on the site.
In May, Thames Valley cops arrested a 29-year-old woman and a 31-year-old man, both from London, after a report that two trespassers had been in the grounds of the Royal Lodge at Windsor Great Park on the afternoon of April 25.