A statement from the palace on Sunday said Prem died of heart failure at Bangkok's Phra Mongkutklao hospital.

Prem was best known for his long-standing devotion to the monarchy, especially the late King Bhumibol Adulyadej, who appointed him to his Privy Council immediately after Prem's eight years as prime minister, and named him head of that powerful advisory body in 1998, a position he held until his death.

Prem is credited with establishing the primacy of the palace in Thailand's power structure, cementing a mutually beneficial alliance with the military.

"He essentially forged and sustained a partnership between monarchy and military with the latter as junior partner up until today," Paul Chambers, a political scientist at Naraesuan University in northern Thailand, said.

Prem was prime minister from 1980 to 1988, ushering in a period of relative stability after a successful pro-democracy uprising against a military dictatorship in 1973, a counter-revolution and coup in 1976 and another coup in 1977.

While most Thai army commanders came to the position through coups, Prem was elected constitutionally by parliamentary vote, though he never ran for office. As prime minister, he weathered two attempted coups and was reportedly the target of several assassination plots.

Critics questioned his devotion to democracy and accused him of encouraging the 2006 coup that ousted elected Prime Minister Thaksin Shinawatra.

He denied such allegations, but in the months preceding the military takeover, he frequently spoke out about corruption and greed in government - a major accusation against Thaksin by his critics - and advised in public speeches to army and navy cadets that their loyalty was to king and country.

The coup set off a sometimes violent battle for power between Thaksin's opponents and his political allies who, despite electoral victories, were forced time and again from office, culminating in another coup in 2014.

Prem was born in the major southern fishing port of Songkhla on August 26, 1920. He attended the prestigious Chulachomklao Royal Military Academy in Bangkok and later US Army schools. He began his military career in 1941 as a second lieutenant in a tank regiment.

Junior officers pushed a reluctant Prem into taking the prime minister's job in 1980, when Thailand was facing an ailing economy and perils on the border with Cambodia, which had been occupied by Vietnamese forces who had driven out the communist Khmer Rouge regime but also sent hundreds of thousands of refugees into Thailand. At the same time, Thailand expanded ties with China and allies in the West, Japan and Southeast Asia.

The border crisis with Cambodia eased over time, and Prem had the good luck to preside over the birth of Thailand's economic boom, which ended only with Asia's devastating 1997 financial crisis.

Prime Minister Prayuth Chan-ocha, the junta leader who seized power in 2014, praised Prem as "a role model for Thais who love the country," according to a statement from a government spokesman.

Never married, Prem leaves no family survivors.

Australian Associated Press