Sultan Qaboos bin Said al-Said. Photo Al Jazeera

Oman has declared three-days of official mourning for long reigning Sultan Qaboos bin Said Al Said, who died after a battle with colon cancer. Flags will be flown at half staff for 40 days as part of the mourning.

Oman’s Sultan Qaboos died on Friday evening, state media said early on Saturday without mentioning a cause of death.

Western-backed Qaboos, 79, had ruled the Gulf Arab state since he took over in a bloodless coup against his father in 1970 with the help of Oman’s former colonial power Britain.

Unmarried Qaboos had no children and had not publicly appointed a successor. A 1996 statute says the ruling family will choose a successor within three days of the throne becoming vacant.

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If they fail to agree, a council of military and security officials, supreme court chiefs and heads of the two consultative assemblies will put in power the person whose name has been secretly written by the sultan in a sealed letter.

A three-day period of official mourning for the public and private sectors has been declared, and , state media said.

Oman state news agency ONA said Qaboos died after “a wise and triumphant march rich with generosity that embraced Oman and extended to the Arab, Muslim and entire world and achieved a balanced policy that the whole world respected.”