The Queen has paid her condolences following the death of His Highness Sheikh Khalifa bin Zayed Al Nahyan.
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A statement released by Buckingham Palace read: "I am saddened to hear of the death of His Highness Sheikh Khalifa bin Zayed Al Nahyan, President of the United Arab Emirates and Ruler of Abu Dhabi."
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It continued: "Your distinguished brother devoted his life to serving the people of the United Arab Emirates and his relationship with its allies and friends.
"He will be long remembered by all who work for regional stability, understanding between nations and between faiths, and for the conservation cause."
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She signed off: "I offer Your Highness my sincere condolences and I extend my sympathy to the people of the United Arab Emirates. ELIZABETH R."
The UAE's state media WAM confirmed the 73-year-old had "passed away to the Lord's side" on Friday morning.
Queen Elizabeth II and His Highness Sheikh Khalifa bin Zayed Al Nahyan in 2013
"The Ministry of Presidential Affairs mourn the people of the UAE, the Arab and Islamic nations, and the whole world. The leader of the nation and the patron of its march, His Highness Sheikh Khalifa bin Zayed Al Nahyan, President of the State, passed away to the Lord's side today, Friday, May 13," WAM said.
"The Ministry of Presidential Affairs announces an official mourning and flags to be flown at half-mast for the late His Highness Sheikh Khalifa bin Zayed Al Nahyan, may God have mercy on him, for a period of 40 days, starting today, and suspending work in ministries, departments, federal and local institutions, and the private sector for 3 days, starting today," WAM later tweeted.
Born in 1948 Sheikh Khalifa was the eldest son of Sheikh Zayed, the first president and founder of the United Arab Emirates.
He had suffered a stroke and had surgery in 2014, leaving him in a mostly ceremonial role in the years following. His brother and Abu Dhabi's crown prince, Sheikh Mohammed bin Zayed, has been handling day-to-day affairs for the Gulf state ever since.
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