Tributes have been paid to Sheikh Sultan bin Zayed Al Nahyan, the President's Representative, who died on Monday.
President Sheikh Khalifa announced the death of his brother in a statement carried by the Ministry of Presidential Affairs, which said the country would observe three days of official mourning, including the lowering of the flags, beginning on Monday.
Sheikh Mohammed bin Rashid, Vice President and Ruler of Dubai, offered his condolences to the Al Nahyan family and the people of the UAE.
"Zayed's sons have eternal fingerprints in the UAE," Sheikh Mohammed said on Twitter.
"Zayed's sons have a different love in the heart of every Emirati. Zayed's sons are partners in the foundation who time will never forget. May God have mercy on Sheikh Sultan."
Sheikh Mohamed bin Zayed, Crown Prince of Abu Dhabi and Deputy Supreme Commander of the Armed Forces, described him as loyal to the nation and one of its national symbols.
"Sheikh Sultan worked with the founder Sheikh Zayed to place the foundation blocks of the unification [of the Emirates] and served his people. May God have mercy on his soul," Sheikh Mohamed said.
Sheikh Sultan's name immediately began trending on social media with users posting tributes and condolences to the Abu Dhabi Royal Family.
Sheikh Hamad bin Mohammed Al Sharqi, Ruler of Fujairah, described Sheikh Sultan as one of the country's "most loyal men", state news agency Wam reported.
''With the death of Sheikh Sultan, the UAE has lost one of its most loyal men, who escorted the late founding father, Sheikh Zayed bin Zayed Al Nahyan, since the early journey of establishing the Union," he said.
"With his departure, we recall the long journey of building our nation, during which Sheikh Sultan accompanied the generation of our founding fathers."
Sheikh Sultan was born in Al Ain in 1955 as the second son of Sheikh Zayed, the UAE's Founding Father.
He was educated at Millfield School in Somerset and attended Sandhurst Military Academy.
He previously held the post of Deputy Prime Minister but was then appointed the President's Representative – a post he held until his death.
Sheikh Sultan was the patron of an annual heritage festival held at Sweihan racecourse in Abu Dhabi.
The Sheikh Sultan bin Zayed Al Nahyan Heritage Festival included camel and saluki races, a camel beauty contest and traditional markets that sold handicrafts and hosted traditional activities.
During the three days of mourning, flags will be flown at half-staff and while regular radio programmes will be replaced with classical music or Quran recitations.
Most live entertainment shows, including radio broadcasts, are also typically cancelled during a nationwide mourning period.
In the past, federal ministries and government departments have been closed, although this has declined in recent years and schools have generally remained open.
After the death of Founding Father, Sheikh Zayed in November 2004, the official mourning period was set at 40 days.
Government departments and schools closed for eight days and the private sector shut for three.