The Queen has sent a message of condolence to the new Emir of Kuwait, following the death of Sheikh Sabah Al Ahmad Al Sabah.
MORE: The Queen and Prince Philip to live apart?
"I was saddened to hear of the death of His Highness Sheikh Sabah Al Ahmed Al Jaber Al Sabah," read the monarch's letter, which was shared on the royal family's official Twitter account.
WATCH: Milestones of the Queen's 25,000-day-long reign
"Your distinguished brother devoted his life to the service of the State of Kuwait and especially its relationship with its allies and friends.
MORE: Prince Charles: shocking fact revealed in official court papers
MORE: 20 surprising facts about the Queen's royal banquets
"He will be long remembered by all who work for regional stability, understanding between nations and between faiths, and for the humanitarian cause. I have deeply valued his friendship towards the United Kingdom, and his memorable State Visit in November 2012.
© Photo: Getty ImagesThe Queen pictured with Sheikh Sabah Al Ahmad Al Sabah
"I offer Your Highness my sincere condolences. I offer also my sympathy to the people of Kuwait. May the long history of close companionship between our two families continue." The message was signed Elizabeth R.
MORE: 24 sweet photos of Kate Middleton with her royal in-laws
The death of 91-year-old Sheikh Sabah al-Ahmad al-Sabah was announced on state television on Tuesday afternoon by the Minister of Emiri Diwan (Court) Affairs, Sheikh Ali Jarrah al-Sabah.
© Photo: Getty ImagesThe Emir of Kuwait passed away at the age of 91
"With the utmost sadness and grief for the Kuwaiti people, the Islamic and Arab world and people of friendly nations, the Emiri Diwan mourns the death of Sheikh Sabah al-Ahmed al-Jaber al-Sabah, the emir of Kuwait, who is now next to God," he said.
It's thought the monarch died at the Mayo clinic in Rochester, Minnesota, where he has been recovering after surgery in July. The cause of death has not yet been revealed. He will be flown back home to Kuwait for burial.
MORE: The real story of Princess Margaret's first love Peter Townsend
Sheikh Sabah had ruled Kuwait since 2006, and steered its foreign policy for more than 50 years. His 83-year-old half-brother, Crown Prince Sheikh Nawaf al-Ahmed, has been named by the cabinet as his successor.
© Photo: Getty ImagesCrown Prince Sheikh Nawaf al-Ahmed has been named his successor
Among those to also pay tribute to the late ruler was Jordan's King Abdullah II, who wrote in Arabic on Twitter: "Today we lost a big brother and a wise and loving leader… who spared no effort for Arab unity."
"The emir was a man of profound compassion, humanity and dignity who all his life worked tirelessly for the best interests of his country and the region," said former UK prime minister Tony Blair.
© Photo: Getty ImagesSheikh Sabah had ruled Kuwait since 2006
"He was deeply respected across the Middle East. He showed courage at the time of the invasion of Kuwait and wisdom in rebuilding his nation afterwards."
Born in 1929, Sheikh Sabah became Kuwait's foreign minister in 1963 and held the position until 1991. He was reappointed in 1992 and served until 2003 when he was named prime minister by his half-brother, Emir Sheikh Jaber al-Ahmad.
He became ruler of Kuwait in 2006 after the death of Sheikh Jaber and following his cousin Emir Sheikh Saad al-Abdullah's decision to step down just nine days into his rule.
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity, royal and lifestyle news delivered directly to your inbox.
