The 39-year-old second son of Dr Sheikh Sultan bin Mohammed Al Qasimi was found unresponsive when emergency services were called to the upmarket address in London shortly after 10:30am on Monday.
The Metropolitan Police said the death is being treated as unexplained, and an initial post-mortem examination failed to identify a cause of death.
Police said no arrests had been made and inquiries were continuing.
A London Ambulance Service spokesman said: “We were called at 10:32am Monday to reports of an incident on Knightsbridge, SW7.
“We sent an ambulance crew and a medic in a car to the scene. Sadly a person was pronounced dead at the scene.”
The spokesman was unable to confirm what the victim was treated for.
The United Arab Emirates-born designer founded an eponymous fashion brand in 2016 and had shown collections at London and Paris fashion weeks.
Three days of mourning have been announced in the UAE following the death of the “great philosopher and artist”, with flags flying at half-mast.
He was laid to rest yesterday in Sharjah. The funeral prayer was held at the nearby King Faisal Mosque.
Photographer Mariano Vivanco was among those paying tribute, calling the designer “my angel”.
His father posted on Instagram: “The arrival of the body of the deceased and prayers upon him will be announced.”
Prince Fahad Al Saud of Saudi Arabia said: “This is so sad, I loved his work. Rest in power.”
The designer moved to the UK from the UAE when he was nine. He was educated at Tonbridge School where he was awarded an art scholarship upon entry, and went on to read French and Spanish at University College London, subsequently completing a degree in architectural studies at the Association School of Architecture, his website said.
He later studied architecture and fashion design at Central Saint Martins in London, and launched his first fashion label, Qasimi Homme, in 2008.
His brother, Sheikh Mohammed bin Sultan Al Qasimi, died of a heroin overdose in 1999. The ruler of the Emirate state of Sharjah found his eldest son dead on the floor of his quarters in the family’s £3 million English manor house in Sussex, an inquest heard.
A statement on the Qasimi website said: “Khalid was praised for his tenacious yet sensitive exploration of social-political issues, particularly pertaining to the Middle East and its sometimes strained relationship with the West, a subject very close to his heart and upbringing.
“His goal was to create ‘a world of beautifully crafted products infused with cultural, social and political undertones to inform and inspire’.
“The design world has lost a great philosopher and artist.”
Qasimi is stocked in 50 stores in 30 cities worldwide.