Camilla, Future Queen, Appears in Rare Portrait with Queen Elizabeth, Signaling Future of Monarchy

The Duchess of Cornwall was "very pleased" to receive the honor, a royal source tells PEOPLE

Camilla, Duchess of Cornwall is celebrating a major royal milestone alongside Queen Elizabeth and her husband Prince Charles.

On Monday, the royal, 74, was formally invested by Queen Elizabeth into the oldest order of chivalry in the U.K.: the Order of the Garter.

In honor of the occasion, the palace released a new portrait of Camilla alongside Charles, 73, and the 96-year-old monarch. The private ceremony took place in the Garter Throne Room in Windsor Castle. She will now formally be acknowledged as a Royal Lady of the Order of the Garter.

Camilla was "very pleased" to receive the honor, a royal source tells PEOPLE.

Dressed in the traditional velvet robes and hat with an ostrich plume, Camilla stood next to the Queen, who wore her Sovereign of the Garter sash and used a cane (she also carried her signature purse!).

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

The photo signals the future of the monarchy. Earlier this year, Queen Elizabeth announced that it was her "sincere wish" that Camilla will become Queen Consort when Prince Charles accedes to the throne.

Prince Charles and Camilla Duchess of Cornwall Garter Day at St George's Chapel
Prince Charles and Camilla, Duchess of Cornwall. Tim Rooke/Shutterstock

The Queen, who has been suffering ongoing mobility issues and used a cane during the service, did not join her family members in the public portion of Monday's royal event. She hosted the private meeting and lunch inside the castle.

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Every June, a grand procession of the knights takes place at Windsor Castle, accompanied by a marching band and Officers of the Order, all in grand ceremonial dress. The ancient order of chivalry was founded by King Edward III in 1348.

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Prince Charles, Prince William, Camilla and Prince Edward did the ceremonial walk to St. George's Chapel at Windsor Castle as Kate Middleton and Sophie, Countess of Wessex looked on.

The Order Of The Garter Service At Windsor Castle

Queen Elizabeth's Platinum Jubilee celebrations last week also doubled as a preview of the future of the monarchy in the hands of Prince Charles.

The Queen limited herself to just a few appearances over the four days of festivities due to her mobility issues and "discomfort," meaning her heir and son Prince Charles took the lead at multiple events. From taking the salute on horseback at Trooping the Colour to leading the tributes at the Platinum Party at the Palace, Charles was in the spotlight.

"[The Queen] sees this as a great opportunity for the transition to be visible," royal historian Robert Lacey tells PEOPLE.

Adds Sally Bedell Smith, author of Elizabeth the Queen: The Life of a Modern Monarch: "She is in the saddle, but this gets people accustomed to [Charles's] future role as King. There was a feeling of celebrating the past and anticipating the new era."

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