Camilla, Duchess of Cornwall appears as we've never seen her before

The Duchess was formally invested into the oldest order of chivalry in the UK, the Order of the Garter. Photo / Getty Images

Camilla, Duchess of Cornwall is celebrating a royal milestone.

Standing alongside the Queen and Prince Charles, Camilla was formally invested into the oldest order of chivalry in the UK, the Order of the Garter.

The royal milestone – which took place in a private ceremony in the Garter Throne Room in Windsor Castle – means the duchess will be formally acknowledged as a Royal Lady of the Order of the Garter.

A source revealed to People Magazine the duchess was "very pleased" to have the honour bestowed upon her and to celebrate the momentous occasion, Camilla, Charles and Her Majesty appeared in a royal portrait.

Camilla, Duchess of Cornwall, Queen Elizabeth and Prince Charles. Photo / Buckingham Palace

Camilla wore an ostrich plume hat while she and Charles both donned traditional velvet robes. The Queen, who continues to be plagued by mobility issues, stood in the middle of the royal couple wearing her Sovereign of the Garter sash and could be seen holding a cane.

The official portrait is both a celebration of Camilla's achievement as well as a signal of the future of the monarchy. It comes after the Queen's announcement earlier this year that it was her "sincere wish" that Camilla will become Queen Consort once Charles accedes to the throne.

Meanwhile, the Duke of York, Prince Andrew was banned from appearing at the ceremony following an intervention by Charles and Prince William. They reportedly told Her Majesty they would "pull out" of the royal event if the disgraced prince was given a public role.

The couple wore traditional velvet robes and hats featuring an ostrich plume. Photo / Getty Images

The Daily Telegraph reported the senior royals feared "backlash" and insisted the Queen make a change of plans. Andrew is thought to have challenged the ban but to no avail.

A source later told the Sun that Andrew was told to stay out of sight "for his own good" following the tense family talks.

However, a source close to the duke insisted that it was his own "personal decision" not to appear in public.

Having been forced to pull out of the service of thanksgiving at St Paul's Cathedral during the Platinum Jubilee celebrations earlier this month after testing positive for coronavirus, he was looking forward to the public appearance and considered a small step on the way to his return to some form of royal life.

He was expected to join the Queen and rest of his family for the investiture in the castle's Garter throne room and the traditional lunch that takes place afterwards in the Waterloo Chamber.