The United Arab Emirates’ long-ailing ruler, Sheikh Khalifa bin Zayed Al Nahyan, has died, the government’s state-run news agency announced in a brief statement.
He was 73.
The UAE’s ministry of presidential affairs announced a 40-day period of mourning and a three-day suspension of work in all ministries and the private sector beginning on Friday.
Flags will be flown at half-mast.
Sheikh Khalifa had long ceased being involved in the day-to-day running of the country, with his brother, Abu Dhabi’s Crown Prince Mohammed bin Zayed, seen as the de facto ruler.
There was no immediate announcement about a successor, although Sheikh Mohammed is in line to inherit the top post.
Sheikh Khalifa, who had rarely been seen in official photos or at public events for years, succeeded his father and the UAE’s founder Sheikh Zayed in 2004.
He suffered a stroke a decade later, keeping him largely out of public sight.
The world’s tallest tower in the UAE’s emirate of Dubai is named the Burj Khalifa after the ruler, whose oil-rich emirate helped bail Dubai out during a financial crisis.
Sheikh Khalifa, the eldest son of the UAE’s first leader after the federation’s formation in 1971, held the most powerful position among the seven semi-autonomous city-states stretching along the shores of the Persian Gulf and the Gulf of Oman.
His role as president derived from his standing as hereditary ruler of Abu Dhabi, the UAE’s largest and richest emirate.
Abu Dhabi serves as home to the federal capital.
Despite its size and vast oil wealth, Abu Dhabi often found itself overshadowed by the glitzy neighbouring emirate of Dubai, the Middle Eastern commercial hub that showcases both the UAE’s bold visions and, at times, debt-fuelled pipe dreams, including a massive palm-shaped man-made island that sits empty years after its creation.
As Dubai’s fortunes began to falter along with the global economy in 2009, Sheikh Khalifa led efforts to protect the federation by pumping billions in emergency bail-out funds into Dubai.
The two emirates do not always see eye-to-eye on foreign policy decisions and compete commercially with one another.
In 2003, Sheikh Khalifa called for the creation of a new airline, Etihad Airways, which competes with Dubai’s successful and much larger carrier Emirates Air.
He increasingly used Abu Dhabi’s oil wealth to attract cultural and academic centres, such as branches of the Louvre museum and satellite campuses of New York University and the Sorbonne.
He also presided over efforts to move the Opec country beyond its reliance on crude oil export revenue with investments in renewable energy research, including plans for a futuristic low-carbon desert city known as Masdar.
Abu Dhabi’s big spending overseas during Sheikh Khalifa’s rule also helped push the emirate, which controls the bulk of the UAE’s oil reserves, out from Dubai’s shadow.
In 2007, the Abu Dhabi Investment Authority, one of the world’s biggest sovereign wealth funds, came to the rescue of an ailing Citigroup with a £6.2 billion cash injection.
Less than two years later, another Abu Dhabi state fund made one of its biggest in a series of headline-grabbing purchases when it paid nearly two billion euros (£1.7 billion) for a 9.1% stake in German car maker Daimler AG, the firm behind Mercedes-Benz.
Sheikh Khalifa, meanwhile, helped boost the UAE’s regional profile with relief missions to Pakistan after devastating floods and by sending fighter jets to the Nato-led mission against Muammar Gaddafi’s regime in Libya in 2011.
Questions were raised during Sheikh Khalifa’s rule about the UAE’s use of foreign military contractors, including one linked to the founder of the former Blackwater security firm, Erik Prince, who moved to Abu Dhabi in 2009.
Mr Prince was involved in a multimillion-pound programme to train troops to fight pirates in Somalia, according to an official who spoke to The Associated Press in early 2009.
Sheikh Khalifa’s image was ubiquitous, gracing every hotel lobby and government office across the country.
But unlike Dubai ruler Sheikh Mohammed bin Rashid Al Maktoum, the federation’s vice president and prime minister, he was rarely seen in public.
A US diplomatic cable made public by WikiLeaks in 2010 uncharitably described him as “a distant and uncharismatic personage”.
Sheikh Khalifa was born in 1948 in the inland oasis of Al Ain, near the border with the sultanate of Oman, and named after his great grandfather, Sheikh Khalifa bin Shakhbout.
In 1969, while the area was still a British protectorate, Sheikh Khalifa was named as Abu Dhabi prime minister and chairman of the emirate’s department of defence, which later became the core of the UAE’s armed forces.
After independence in 1971, he became defence minister along with other roles.
Later, the title of supreme commander of the armed forces was assumed by Sheikh Mohammed.
Although the UAE’s ruling sheikhs hold near absolute power, Sheikh Khalifa began an experiment with elections by allowing limited voting — by a hand-picked electorate — for half the members of a 40-seat federal advisory body in 2006.
Subsequent rounds of elections in 2011 and 2015 failed to attract even two out of five of those given a chance to vote.
The UAE saw none of the Arab Spring street protests that shook other parts of the region, though in the wake of that unrest, Sheikh Khalifa oversaw tightening crackdowns on Islamists and other activists, drawing criticism from international rights groups.
The UAE also supported efforts in the region to quash the Muslim Brotherhood, including in Egypt.
He was believed to be among the world’s richest rulers with a personal fortune estimated by Forbes magazine in 2008 at £15.6 billion.
He built a palace in the Seychelles, an island-chain nation in the Indian Ocean, and faced complaints there about causing water pollution from the construction site.
After he fell ill, it fell upon his half-brother and designated successor, Sheikh Mohammed, to handle many of his duties, often in conjunction with Dubai’s Sheikh Mohammed.
The transition went largely unnoticed, as many Emiratis and foreign diplomats long assumed that the crown prince was a central power broker in the UAE’s leadership.
In September 2014, the Emirates became one of the most prominent Arab participants in US-led airstrikes against the so-called Islamic State militant group in Syria, deploying its first female air force pilot on the initial raid.
Those sorties were followed by a muscular intervention into Yemen as part of a Saudi-led coalition on the side of the impoverished country’s internationally recognised government against Shiite rebels who had seized the capital of Sanaa and other areas.
The UAE deployed thousands of troops, 52 of whom were killed in a September 2015 missile attack on their base — the heaviest military loss in the country’s history.
Sheikh Khalifa’s personal life was not much in the public eye.
Like many in the Gulf, he was passionate about the traditional sport of falconry and was said to enjoy fishing.
He is known to have had eight children — two sons and six daughters — with his first wife, Sheikha Shamsa bint Suhail Al Mazrouei.
He is also survived by several grandchildren.