For Marshall, though, Bhumibol is little more than a stooge of the military and business elite. His revelatory new book argues that the king has been deliberately elevated to his exalted position so that the traditional ruling classes can maintain their hold on power while denying true democracy to their fellow Thais.

Yet more controversially, Marshall believes the political turmoil of recent years is intimately connected to the question of who will succeed the 86-year-old ailing sovereign, who has spent much of the past five years in hospital. Bhumibol's official heir is Crown Prince Vajiralongkorn, more noted in Thailand and elsewhere for his playboy image than his regal status.

He is also known for his ties to Thaksin Shinawatra, the oligarch and exiled former prime minister whose populist policies have made him a hero to the rural poor, but who is hated by the establishment for his alleged corruption and dictatorial ways. Marshall's thesis is that the prospect of the next monarch being so close to the man who has empowered an electorate Thailand's elite has long ignored is unthinkable. They would rather see the crown prince's sister, Princess Sirindhorn, succeed to the throne.

Marshall's material, written in a clear, unfussy style, is incendiary, especially considering Thailand has some of the strictest lese majeste laws in the world. Just talking or writing about the king, his wife Queen Sirikit, or the crown prince - let alone criticising his rule or role in Thai politics and society - can result in a prison sentence. Twenty-two people are in custody awaiting trial on lese majeste charges.

No one knows the power of the lese majeste laws more than Marshall. A Reuters journalist for 17 years, the 43-year-old Scot resigned in June 2011 after the agency refused to publish a series of his articles on the Thai monarchy. Much of the information for those stories came from hundreds of US diplomatic cables that were released by Wikileaks. Despite having a Thai wife, Marshall has been unable to return to Thailand since.

Marshall reveals how pliant and essentially powerless Bhumibol has been throughout his reign. From the 1950s onwards, the idea of a paternalistic sovereign overlooking his kingdom with a benevolent air has been especially promoted by the military. They did so with the active encouragement of Washington, which regarded Thailand as a bulwark in Southeast Asia against the communists of Vietnam, Laos and nearby China.

That propaganda continues today. Since the May coup, the NCPO has detained scores of its opponents without trial, cracked down further on press and social media freedom, while banning public gatherings of more than five people. Its justification for some of those repressive moves is that they are for the sake of the monarchy, another example of how Bhumibol has been co-opted in the battle for who runs Thailand.

There is little doubt it is a civil war in all but name.

Ever since Thaksin's first political party, Thai Rak Thai (Thais Love Thais), won a crushing victory in the 2001 election, the telecommunications tycoon has been regarded as the ultimate threat to the Thai establishment. Thaksin was the first senior politician to recognise that the sheer numbers of farmers in the northeast of the country ensured they held the key to long-term power. He tailored his policies to meet their needs, thus rendering the opposition Democrats - the traditional party of the ruling elite - all but unelectable.

The metropolitan middle classes, though, were scandalised by the allegations of widespread corruption that characterised the Thaksin years, while moderates were left uneasy by his disdain for the rule of law. When Thaksin launched a war against drugs in early 2003, close on 2,500 people were killed in just three months.

Far more worrying to the royal circle was Thaksin's growing relationship with Vajiralongkorn. He is believed to have lent the crown prince money, earning him the enmity of the king. That meant little to Thaksin - as long he stayed prime minister and the crown prince the official heir. Ralph Leo Boyce, US ambassador to Thailand between 2004 and 2007, noted in one cable that "the king will not be around forever and Thaksin long ago invested in crown prince futures".

A coup in 2006 was designed to end Thaksin's dominance of Thai politics. But the government installed by the military only exacerbated the nation's political divide. The anti-Thaksin camp became known as "yellow shirts", a colour long associated with the king, while his supporters were dubbed "red shirts". Clashes in Bangkok in 2010 between the rival groups and the army left at least 90 people dead.

Nor did the takeover end Thaksin's influence. While it did ultimately drive him into exile to avoid corruption charges, and Thai Rak Thai was banned from politics by the courts, Thaksin rebounded quickly. A new party, Pheu Thai, was formed, and won the 2011 election, with Thaksin's younger sister, Yingluck Shinawatra, as prime minister.

May's coup brought an end to Pheu Thai's rule, but Thailand's future is deeply uncertain and, for Marshall, will remain so until the king passes away. Even then, the possibility of a contested succession will come as a huge shock to a population so used to having King Bhumibol on the throne.

While Marshall's book would benefit from more discussion of the growing grass-roots opposition to the establishment - Thaksin's most lasting legacy may be politicising a formerly placid population in just a decade - it is still a timely analysis of Thailand's dysfunctional system of government.

It is a brave book too. The Land of Smiles has long been adept at shrouding its less attractive side from public view, whether it is the ongoing insurgency in the deep south of the country that pits ethnic Malay Muslims against the Buddhist Thai state, or the exploitation of migrant workers. Marshall throws a harsh light on the political role played by the royal family in a country where it has long been allowed immunity from criticism, and that is a unique achievement.