An armed man has been arrested after breaking into the grounds of Windsor Castle on Christmas Day.

Officers from Thames Valley Police (TVP) and the Metropolitan Police responded to a security breach about 8.30am (7.30pm AEDT) on Christmas Day within the grounds of the castle in Berkshire where the Queen is in residence.

TVP said a 19-year-old man from Southampton was arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon.

The Sunday Mirror said the arrest came after security controllers monitoring CCTV spotted a man carrying what appeared to be a crossbow.

The Mail on Sunday said sources suggested the man was armed with a crossbow and used a rope ladder to scale a metal fence.

Police did not confirm what the weapon was and it is not known where exactly the man was on the grounds and how close he got to the castle itself.

The man, who is in custody, was not in any buildings on the estate and “security processes were triggered within moments of the man entering the grounds”, police said.

The Queen spent Christmas Day at Windsor Castle and was expected to have lunch with the Prince of Wales and the Duchess of Cornwall, and the Earl and Countess of Wessex.

Charles, Camilla, Edward and Sophie, along with their children Lady Louise and James, Viscount Severn, attended a church service in St George’s Chapel, in the castle grounds, at 10.45am on Christmas morning.

“An investigation is ongoing following this incident and we are working with colleagues from the Metropolitan police,” TVP Superintendent Rebecca Mears said.

“The man has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon. He remains in custody at this time.

“We can confirm security processes were triggered within moments of the man entering the grounds and he did not enter any buildings.

“Members of the royal family have been informed about the incident. We do not believe there is a wider danger to the public.”

A Buckingham Palace spokesman declined to comment.

In May, police 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.

Police said there was no risk to any individual on the site.

Officers were also called to the Duke of York’s home after a woman reportedly talked her way into his mansion on April 19.

A 43-year-old woman spent 20 minutes in the grounds of Royal Lodge after telling security guards she was having lunch with the Queen’s son, according to the Sun newspaper.

She later made her way into the home and was 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, the newspaper reported.

The article claimed the woman had arrived in Windsor in a taxi and had even persuaded the duke’s security guards to pay her fare.

It is not known if Andrew was at the home he shares with his former wife Sarah, Duchess of York.

Police said the woman was stopped and later sectioned under the Mental Health Act but there was no risk to anyone on the site.

AAP

Get all the latest Canberra news, sport, entertainment, lifestyle, competitions and more delivered straight to your inbox with the Canberra Daily Daily Newsletter. Sign up here.