James Middleton Announces Death of His Dog Ella After 'Short Illness': 'Going to Miss Her Terribly'

James Middleton/Instagram

James Middleton is mourning the death of his beloved dog Ella.

Kate Middleton's younger brother shared the sad news on Instagram Friday, posting two black and white photos of the cocker spaniel.

"It is with great sadness that I announce my dear Ella has passed away," James, 35, began the caption. "For 15 years Ella has been at my side, from my darkest days to my happiest. I'm going to miss her terribly."

"Ella had a very short illness, she slipped away in my arms at home and is now buried in the garden alongside [the late family dog] Tilly," he continued. "I took this picture just a few hours before she died."

"I'm just about holding it together to write this, and despite the fact I knew this day was coming, it doesn't make it any easier," James wrote. "Goodnight my darling Ella, [my wife] Alizee and the dogs will take good care of me."

RELATED: James Middleton Honors Queen Elizabeth by Giving New Puppy a Royal-Inspired Name

The first photo showed Ella sitting in the grass, staring straight into the camera, followed by a second shot of James lovingly holding her in his arms.

Ella was the first dog James owned in adulthood, and he previously credited the sweet spaniel as his strength while struggling with depression. While appearing on the BBC's Sunday Morning Live in 2020, James revealed that Ella came along to his therapy sessions, which made all the difference.

"I got this diagnosis and Ella came to all of my therapy sessions, which I was reluctant to go from the start," he said at the time. "But with her by my side, I really became confident in understanding that I had a mental illness."

"I would go so far as to say that I owe Ella my life, and she doesn't know that," he added.

Kirsty O'Connor/Getty Images James Middleton and his dogs

RELATED: Kate Middleton Shares the Creative Way She Tries to 'Squeeze in Exercise' Before Kids Go to School

When James launched his all-natural pet food company in 2020, he named it Ella & Co. in her honor.

"I founded Ella & Co, a happiness and wellbeing company for dogs, as a way of giving back to my dogs that have done so much for me," he said on the website, where he celebrated Ella as his "teacher, therapist and best friend."

James Shaw/Shutterstock

In a nod to their tight bond, James previously brought Ella as his plus-one to the GQ Men of the Year Awards, and even credited the dog for introducing him to his wife, Alizee Thevenet.

James and Alizee met in 2018, got engaged the following year, and tied the knot in France in 2021. In an essay for The Telegraph, James opened up about how Ella brought them together.

James Middleton/Instagram

Can't get enough of PEOPLE's Royals coverage? Sign up for our free Royals newsletter to get the latest updates on Kate Middleton, Meghan Markle and more!

"The two of us were at the South Kensington Club in Chelsea," he wrote in 2021. "Ella was lying at my feet under the table; realising she might want some water, I trusted her to take herself to the water bowl across the terrace. However, she made a beeline for Alizee. Rather embarrassed, I went over to apologize and bring Ella back."

According to the story, Alizee thought James was a server, and ordered a drink while petting the dog.

"Little did I know, I had just met my future wife, all thanks to Ella," he said. "If I hadn't trusted Ella, I wouldn't have brought her to the South Kensington Club and she wouldn't have been able to say hi to the woman who became my fiancée."

James and Alizee own six other dogs — Zulu, Inka, Luna, Mabel, Nala, and Isla.

In another family tie, Ella was the mother of Lupo, the black cocker spaniel Prince William and Kate Middleton owned and loved for nine years, until the dog's death in November 2020.

"Very sadly last weekend our dear dog, Lupo, passed away," the Prince and Princess of Wales said in a statement at the time. "He has been at the heart of our family for the past nine years and we will miss him so much."

PEOPLE previously confirmed that Prince William and Princess Kate got another new puppy from James before Lupo died. The black cocker spaniel joined Princess Charlotte in one of the young royal's 7th birthday portraits last year, where her name was revealed as Orla.

The name is especially fitting for a royal pet — in Celtic, Orla means "golden princess."