Thailand's King, 88-year-old Bhumibol Adulyadej, has died after a reign that lasted more than 70 years. His health had waned over the past two years and he was breathing with the aid of a ventilator.

The king known as Rama IX "was a unifying figure," said Paul Chambers, director of research at the Institute of Southeast Asian Affairs in Chiang Mai. "The appearance that he gave throughout his rein was at least publicly very positive. A father a figure, a loving figure, and people will be very very sad. With his passing ... it is going to be a very trying time for Thailand."

In a televised speech the the country's Prime Minister, Prayut Chan-o-cha, a retired Royal Thai Army officer and head of the country's ruling National Council for Peace and Order military junta, said that Prince Maha Vajiralongkorn will succeed the king.

PM Prayut said that Adulyadej's death will be observed with a year of mourning in the country. For the next month the country's flags will fly at half-mast and all events will be "toned down."

For the past week hundreds of Thai royalists have held vigil outside Siriraj Hospital in Bangkok following an official announcement Sunday that the king's health was unstable following hemodialysis to treat his kidneys.

An official statement about the king's health holds great significance in Thailand because laws bar discussing the king's health in public.

On Tuesday PM Prayut held a special ceremony for the King along with other officials. All official government engagements were cancelled Wednesday.

The UK's ambassador to Thailand, Brian Davidson, said that he was "saddened to hear of the death" of Thailand's ruler and that he will be missed.

For many in the country the king has been a symbol of stability. In just the past decade Thailand has seen two military coups, historic conflicts in neighbouring countries Vietnam and Cambodia, violent demonstrations, and deep political division.

Thailand has been a constitutional monarchy since 1932 when a revolution by civilian and military officials overthrew the absolute monarchy.

The king was last seen in public on 11 January. He has spent much of the last few years in and out of hospital. This year news trickled out that he had received treatment for excess fluid in his spinal cord and brain as well as a blood infection and a lung inflammation.

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Thailand's King Bhumibol Adulyadej is pictured in 1946 and in 2010 Reuters