President of the United Arab Emirates has died at the age of 73, here’s what we know about the long-ailing ruler.

Since the death of the country’s leader, 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 also be flown at half-mast.

Boris Johnson pays tribute to Khalifa bin Zayed Al Nahyan

In a statement, UK Prime Minister Boris Johnson said: “I am deeply 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.”

What did Khalifa bin Zayed Al Nahyan do?

Born in 1948 in the inland oasis of Al Ain, near the border with the sultanate of Oman, Sheikh Khalifa was 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.

The eldest son of the UAE’s first leader, he 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.

His role as president derived from his standing as hereditary ruler of Abu Dhabi, the UAE’s largest and richest emirate.

Sheikh Khalifa, who had rarely been seen in official photos or at public events for years, 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.

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.

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.

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.

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.

Who will be next UAE President?

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.