Brunei's Prince 'Abdul 'Azim died of multiple organ failure due to an autoimmune disease, his younger brother revealed on Tuesday.
Prince 'Abdul Mateen said in an Instagram post that his brother was diagnosed with severe systemic vasculitis earlier this year, a condition characterised by inflammation of blood vessel walls.
"At the same time, he was also coping with bipolar disorder which made the fight harder," said Prince 'Abdul Mateen.
"Eventually, my brother succumbed to multiple organ failure caused by persistent infections due to the autoimmune disease."
Prince 'Abdul 'Azim died last Saturday and was laid to rest at the Royal Mausoleum on the same day. He was 38.
Brunei is now in a seven-day mourning period.
Prince 'Abdul Mateen, 29, said he shared the details of his brother's death as that was what the late prince would have wanted. "He was a big advocate of bringing awareness to different causes," said Prince 'Abdul Mateen.
The post was accompanied by a black and white childhood picture of them having fun.
Paying tribute to his elder brother, Prince 'Abdul Mateen said there was never a dull day when "Abang Azim" was around.
"He loved to always prank us, joke around and never took life too seriously. His laughter was so contagious that regardless of whether the thing was funny or not, everyone would end up laughing too. He definitely lived life to the fullest.
"I spent every day of my childhood with my brother - he taught me so much and made me who I am today and for that I am eternally grateful."
Fourth in line to succeed the throne of Brunei, Prince 'Abdul 'Azim was the second-born prince of Brunei's ruler, Sultan Hassanal Bolkiah.
He was born in Bandar Seri Begawan on July 29, 1982, and studied at International School Brunei, Singapore's Raffles Institution and Oxford Brookes University in England.
NBC news reported that the late prince had a London-based production company called Daryl Prince Productions and that he had a stint as a Hollywood movie producer.
His credits, under the name Azim Bolkiah, included the 2014 movie You're Not You starring Hillary Swank, and 2018's The Happy Prince with Rupert Everett and Colin Firth.
Foreign news outlets have often reported on Prince 'Abdul 'Azim's glamorous lifestyle, splashing pictures of him with numerous A-list celebrities.
Over the years, those he had rubbed shoulders with included Mariah Carey, Elizabeth Hurley and Michael Jackson.
His celebrity friends were quick to pay tribute to him.
American actor Kevin McHale described the prince as a "true light" on Twitter, saying that he was always generous and welcoming, while on Instagram, veteran English actress Joan Collins uploaded a picture of herself at a restaurant with Prince 'Abdul 'Azim and model and actress Jerry Hall.
"Rest In Peace #princeazim - you were a good friend to me and everyone that was lucky to know you. #lifegiver #angel #ripprinceazim," she wrote.
Singer Janet Jackson took to her Instagram story to post: "Azim, I love you too and insyallah (God willing) no more pain. In the name of the most high, the most merciful, He will take care of you now. Ameen."
As much as he was known for his glamorous lifestyle, Prince 'Abdul 'Azim was also known for his support of charitable causes.
The Times newspaper reported that he designed unisex weekend bags for luxury fashion label MCM, with the proceeds going to the Make-A-Wish Foundation charity in Britain for critically ill children.
In 2011, he lent his support to a charity gala in Brunei to raise awareness of autism.
"I think it is important for me to be here and show my support. I think you have to look after what is at home before you do anything else. You have to do what you can with what you have," he told The Brunei Times in an exclusive interview.