The Emirate of Sharjah, the third biggest city in the UAE, announced the death in a statement on Tuesday, with funeral prayers held for Prince Al Qasimi on Wednesday.
Sheikh Dr Sultan bin Muhammad Al Qasimi announced three days of national mourning in with flags flying at half mast.
In a social media post, Sheik Khalid's father said his son "is in the care of God".
The city of Sharjah and Sheikh Dr Sultan bin Muhammad al Qasimi received condolences from leaderships across the Middle East, including Crown Prince Mohammad bin Zayed Al Nahyan of Abu Dhabi as well as the heads of state from Saudi Arabia and Jordan, CNN notes.
Prince Khalid Al Qasimi was the Emir's last surviving son, CNN reports.
The Arab Prince moved to London at the age of nine, where he was educated at Tonbridge School.
He studied French and Spanish at University College London before enrolling in art school at the prestigious Central Saint Martins.
Prince Khalid launched his eponymous fashion label Qasimi in 2008 and had his designs shown at London Fashion Week.
He was often pictured with British models and society beauties, including Brazilian-British catwalk queen Alice Dellal and Pixie Geldof.
Qasimi's celebrity fans included Lady Gaga and Cheryl Tweedy, News.com.au reports.
The ruler of oil-rich Sharjah owns a country estate in Sussex, where, in a cruel twist of fate, Sheik Khalid's older brother was found dead from a heroin dose in 1999, aged 24.
Prince Khalid Al Qasimi was acquainted with the British royal family, and even played against Prince Harry in a charity polo match in 2003.
Last night, Prince Khalid's friend and former business partner Elliott Frieze said: "He was incredibly hardworking and talented. He was a wonderful person."