Security officers saw a man scaling the perimeter fence of the monarch's Berkshire home before armed police scrambled to the scene. Officers from Thames Valley and the Metropolitan Police were informed about the incident at 8.30am as the Queen, 95, prepared to welcome the Firm for their festive celebrations.

       A palace source said: "There is a chill in the air over what could have been."

       The Mirror reports Princess Diana's former bodyguard Ken Wharfe also has concerns about Her Majesty's safety.

       He said: "The Queen’s safety is clearly under threat.

       "It’s often been thought that Windsor is particularly at risk to intruders due to the make-up of the estate and the severity of this event should really cause alarm bells to start ringing.

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       "This individual was caught in time but any system could always be beefed up as any other scenario could be tragic."

       But the former head of the royal protection unit Dai Davies praised the officers at the scene for averting a "disaster".

       He said: "The officers did their job and averted a disaster."

       However, Mr Davies also stressed the need for the Queen's Berkshire home to sort out its problem with trespassers.

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       "Windsor is having an issue with intruders and needs to get to the bottom of it before something more sinister comes to pass," he said.

       The Mirror claims five intruders have got into the royal grounds in 2021.

       The man, 19, from Southampton, was detained under the Mental Health Act and arrested on suspicion of breach of trespass of a protected site and possession of an offensive weapon.

       However, while crossbows can be lethal, they are legal as they do not require a licence.

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       The Metropolitan Police confirmed yesterday it has now taken over the investigation.

       They explained: "Security processes were triggered within moments of the man entering the grounds and he did not enter any buildings.

       "Following a search of the man, a crossbow was recovered."