President of the United Arab Emirates (UAE), Sheikh Khalifa bin Zayed Al-Nahyan is dead.

According to reports, he died aged 73 on Friday, after battling illness for several years.

The president of the oil-rich Gulf state, is likely to be replaced by his half-brother, Abu Dhabi Crown Prince Mohammed bin Zayed, who was already seen as the UAE’s de facto ruler.

“The Ministry of Presidential Affairs has mourned to the UAE people, Arab and Islamic nations and the world the death of President His Highness Sheikh Khalifa bin Zayed Al-Nahyan,” the official WAM news agency tweeted.

The ministry announced 40 days of mourning, with flags at half-mast from Friday and work suspended in the public and private sector for the first three days.

Sheikh Khalifa took over as the UAE’s second president in November 2004, succeeding his father as the 16th ruler of Abu Dhabi, the richest of the federation’s seven emirates.

He has rarely been seen in public since 2014, when he had surgery following a stroke, although he has continued to issue rulings.

Meanwhile, the cause of death was not immediately released.