Riyadh has announced that the heir to the Saudi Arabian throne, Crown Prince Sultan bin Abdulaziz al Saud, has died after a seven-year struggle with cancer.

Prince Sultan, who was in his eighties, was the half-brother of the Saudi ruler King Abdullah bin Abdulaziz. He was also minister of defence and aviation.

He had been on a visit to the US for medical tests, and is reported to have died at a New York hospital. He had an operation in New York in July.

His death was announced on 22 October in a statement from the King’s court, which said: “With deep sorrow and sadness the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz mourns the death of his brother and his Crown Prince Sultan… who died at dawn this morning Saturday outside the kingdom following an illness.”

Prince Sultan had been the kingdom’s minister of defence and aviation since 1963 and throughout that period oversaw huge levels of investment in Saudi Arabia’s military. He was also involved in the setting up and development of the national airline, Saudia.

His death means 78-year-old Prince Nayef, a full brother of King Abdullah, is most likely to become next in line to the Saudi throne. Nayef has been Saudi Arabia’s interior minister, in charge of the security forces, since 1975. In contrast to King Abdullah, who is seen as a cautious reformer, Prince Nayef is believed to be closer to conservative Wahhabi clerics.

In 2009, after Prince Sultan fell ill, King Abdullah named Nayef as his second deputy prime minister, traditionally the post of second in line to the throne.

However, the King has also established the Allegiance Council, known as Al-Hayaat al-Bayaa in Arabic, a succession council, made up of his brothers and nephews. It is expected to meet for the first time to determine who will be named as next in line to the Saudi throne.