Prince William accompanied his father to the event.
Prince Charles played a very important -- and history-making -- role at the Opening of Parliament on Tuesday. The 73-year-old Prince of Wales delivered the Queen's Speech at the event after Queen Elizabeth II was unable to attend due to episodic mobility problems.
Prince Charles' wife, Camilla, Duchess of Cornwall, and his eldest son, Prince William, were on hand to support him while he gave the Queen's message to those in attendance. This was only the second time in Queen Elizabeth's 70-year reign that she did not deliver the message herself.
While Prince Charles has been at his 96-year-old mother's side during the Opening of Parliament in the past, Tuesday was the first time he spoke on her behalf. It also marked Prince William's first time at the ceremonial event.
During the speech, Queen Elizabeth's Imperial State Crown was displayed on a red velvet cushion in between Prince Charles and Prince William. While discussing the administration's objectives and proposed future legislation, Prince Charles used the phrase "Her Majesty's government," and ended the 11-minute talk with a message from his mother.
"Her Majesty prays that the blessings of Almighty God will rest upon your counsels," he said.
"God Save The Queen" later played as Charles and Camilla left the event in royal cars.
Following Tuesday's history-making moment, ET spoke with Royal expert, Katie Nicholl, about the significance of Charles standing in for the Queen.
"It is a huge deal having the Prince of Wales stand in for the Monarch, because one of the most important constitutional roles of the Queen is to deliver the Queen's speech at the start of each Parliamentary session as the head of state. It is her most important constitutional role," Nicholl shared, before noting the Queen's stellar acceptance rate over her 70 years in power.
"The Queen hasn't missed the State of Parliament for 59 years," Nicholl added. "She has always attended it except for two occasions when she was pregnant with Prince Andrew and then with Prince Edward. I think that gets you an idea of just how important this constitutional role is to her."
It was announced on Monday that Queen Elizabeth would have to miss the Opening of Parliament. "The Queen continues to experience episodic mobility problems, and in consultation with her doctors has reluctantly decided that she will not attend the State Opening of Parliament tomorrow," a statement from Buckingham Palace said. "At Her Majesty's request, and with the agreement of the relevant authorities, The Prince of Wales will read The Queen's speech on Her Majesty's behalf, with The Duke of Cambridge also in attendance," the statement continued.
The decision was a last minute one, Nicholl explained, adding that the Queen wanted to wait "until the last possible moment" to make the announcement, in hopes that she would have been able to deliver the annual address.
"The news came up from Buckingham Palace last night. So, it was as much as an 11th hour decision as the Palace usually makes. It is very clear to me that the Queen really wanted to wait until the last possible moment to make that announcement because she would be hoping that she would have been able to go through with today," Nicholl said. "I think it would have been a very difficult decision for her and a very painful decision. This is a very important occasion. The Queen doesn't like to let anyone down, and I think this would not have been an easy decision, and I think with there being any chance for her to have been there, she would have wanted to."
While she was unable to attend in person, Nicholl shared that the Queen did watch Charles deliver the speech from her sitting room at Windsor Castle.
"It is my understanding that the Queen watched the speech delivered by her son from the comfort of her sitting room at Windsor Castle," Nicholl revealed. "It probably wouldn't have been an easy thing for her to watch. I think the Queen would have wanted nothing more than to be able to deliver her government speech herself, but Charles did a very good job."
Though Nicholl noted that the Queen watched with pride, she added that the moment was likely a difficult one, as the Queen had every intention of delivering her speech Tuesday.
"He was very poised, very composed, his elocution, as always, was was superb and he stood in and did a brilliant job for the Queen, but she would have absolutely been watching," Nicholl continued. "She would have taken, I'm sure, great pride in watching Charles deliver it, but it would have been difficult for her because she knows as Queen, as head of state, this was the Queen's Speech."
While Her Majesty has been notably absent from royal affairs in recent months, Prince Charles said on Tuesday that she is looking forward to her upcoming Platinum Jubilee in June and the Commonwealth Games which are set to take place in August.
As far as how Tuesday's absence will impact the Queen's appearance at her upcoming Platinum Jubilee, Nicholl said it's unclear how much of the Queen we'll see.
"I think today does raise the big question as to how much we are going to see the Queen undertake public engagements from now, on and of course with the Platinum Jubilee celebrations coming up in just a few weeks time, we do wonder how much we're actually going to see the Queen," Nicholl said. "We know it's her intention, and obviously her wish to be there on the Buckingham Palace balcony."
She continued, "I think this expectation that we're going to see the Queen very visible and very high profile during the Platinum Jubilee is a little unrealistic, and I think people do forget, because she is so remarkable, because up until now she has always been so robust and she's enjoyed such good health -- we do forget she's 96, we forget that earlier this year she suffered from Covid, a year ago she lost her husband -- I think all of these factors have taken their toll, and she is now, I think, showing her age and certainly, beginning to feel her age."