The Duke and Duchess of Sussex were notably absent from the ceremony.
Meghan Markle and Prince Harry weren't in attendance Saturday during King Charles III's Trooping the Cololur celebration.
While the monarch was joined by other immediate members of the royal family for the occasion -- which marks his official birthday celebration -- the Duke and Duchess of Sussex were home in California.
A source told ET that the couple "were not invited" to the festivities.
On Saturday, the monarch celebrated his official birthday with the Trooping the Colour ceremony. For the occasion, which has been celebrated for 260 years, Charles rode horseback. The move from the king was the first time a royal rode horseback for the occasion since 1986 -- when his mother Queen Elizabeth II did it.
The day was full of pomp and circumstance as 1500 soldiers, 300 horses and 400 musicians came together as the royal and his family rode from Buckingham Palace along the Mall to the Horse Guards Parade. All while royal watchers looked on to celebrate the occasion.
The king's family members had different roles throughout the day. Prince William was dressed in full regalia -- and a bearskin hat -- as he worked as the Colonel of the Welsh Guards. Princess Anne also appeared on horseback. Kate Middleton, her children and Queen Camila all rode in a horse-drawn carriage.
Not included in those festivities are Harry and his family, who stepped down as senior members of the royal family in 2020. The last time Harry and Megan appeared at the Trooping the Colour was in 2019.
"The relationship between Prince Harry and his family remains strained," the source told ET. "He didn't see either Prince William or King Charles while he was in London testifying."
As the source mentioned, Harry was recently in the U.K. to testify in a tabloid case against Mirror Group Newspapers. While in court, Harry outlined how the British media company's stories about his personal life have impacted his relationships, mental health and safety.
Following Harry's testimony, royal expert Katie Nicholl told ET that the Duke of Sussex's family has been apprehensive about him speaking out.
"My sources tell me that [Charles] and [William] were very wary about Harry going into the witness box," Nicholl said. "I think when you see what came out of Harry's evidence and his witness statement, you can understand why."