Charles, 58, took to his Instagram Stories with a clip of ducks flying overhead as he made a trip to the lake on the grounds of Althorp House where Diana's grave stands.
WATCH: Charles Spencer says Princess Diana's grave is 'an oasis of calm'
Captioning the post, he penned: "Duck fly past," whilst also capturing the stunning surroundings where it is thought Diana was laid to rest.
The Earl also shared a special photo on his Instagram feed to mark the poignant day.
Charles shared the poignant update with his fans on Instagram
Captioning a picture of a plaque set in the ground in honour of Princess Diana which he found walking through a park in London, he penned: "Yesterday I was crossing a park in London when I suddenly came across this in the pathway - one of the many that stud parts of the capital, in Diana’s memory.
"Amazing to think of the enduring impact of a woman who died so young. Today - 25 years on - would have been her 61st birthday."
Fans flocked to leave their messages for the Earl in the comments of the post. One fan penned: "I miss her so much. She is still an amazing influence."
Princess Diana is thought to be buried on the grounds of the Spencer family Estate
A second added: "Always remembered - on her birthday and everyday. The unique Diana whose beauty will never be extinguished from our minds."
A third wrote: "Happy heavenly birthday to her. Such an amazing lady x." A fourth said: "Always in our thoughts but especially today on her birthday," with three red love hearts.
Other fans shared their heartbreak on the post, with one fan writing: "I still remember that day and that moment when I heard the news," with a second saying: "A huge loss to all x."
Princess Diana passed away in 1997
Despite the sad anniversary of his sister's passing this month, in June, Charles and wife Karen celebrated their eleventh wedding anniversary.
Proposing to his now wife after less than a year of dating, the Earl presented Karen with a very sentimental ring, opting for a stunning trilogy engagement ring with three round diamonds on a silver band, which was actually a family heirloom.
It was once given to Charles' great grandmother, Countess Margaret Spencer, on her wedding day to 6th Earl Spencer, Liberal politician Charles.
On the tenth anniversary of their engagement in 2021, Karen marked the special milestone with a photo on Instagram, cuddled up to the Earl.
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