"He was not the same after that. He had been unwell for some time. It's been a difficult time for Jack losing his father before the Christening," a source told the Daily Mail.
In Tuesday's edition of the UK's Daily Telegraph, the Brooksbank family placed a notice that read: "Brooksbank, George. Died on 18th November. "Loved all the good things in life but especially his wife, friends, boys, daughters-in-law and grandchildren.
"Private family cremation, memorial details to be announced."
George battled COVID-19 last year during some of the worst outbreaks in the UK, but Eugenie and husband Jack shared a positive update on his condition in June 2020.
At the time, the royal took to Instagram with a moving message dedicated to the NHS staff who helped treat George when he was diagnosed with the virus.
"The COVID-19 pandemic has been a difficult time for so many and I want to say a HUGE thank you to every frontline worker who have kept us all safe and well, whilst sacrificing so much," she penned.
"In particular, I'd like to send an enormous thank you to all the teams who saved my father-in-law, George's life during his stays at St Mary's, Chelsea and Royal Brompton Hospital after he contracted the virus."
The princess revealed that her father-in-law was extremely grateful to the team that cared for him, telling her: "I want to offer profound thanks to everyone who was involved.
"It was a real eye-opener for somebody who has not been in hospital before for any length of time. I certainly owe them my life."
Eugenie went on to add her own words of gratitude, writing: "I can't begin to express the thanks I have to you all for bringing him back to our family, my husband Jack, his brother Tom and his beloved wife Nicola. We are forever grateful. ❤️"
George seemed in good spirits in two photos Eugenie shared at the time and lived for well over a year after his COVID-19 battle.
WATCH: Pregnant Princess Eugenie makes a surprise appearance with a poignant message
His passing came just days before Eugenie and Jack christened their son August in a joint ceremony with Zara and Mike Tindall's son, Lucas Philip Tindall.
The two royal children where christened in a small, private ceremony attended by the Queen, the Duke and Duchess of Cambridge and other key royal figures.
Prince Charles, who had recently travelled to Jordan as part of a royal tour, is also believed to have brought back Holy Water from the River Jordan that was used in the ceremony.
This article first appeared on our sister site, NowToLove.