Thai police have banned a British journalist's book about politics in Thailand for "defaming" the monarchy, an official said Thursday, in a country with one of the world's strictest lese majeste laws.

The sale and distribution of "A Kingdom in Crisis" by freelance journalist and author Andrew MacGregor Marshall, formerly based in Bangkok, has been banned in Thailand a month after it was published by London-based Zed Books.

"The content insults, defames and threatens Thailand's monarchy," said national police chief Somyot Poompanmoung in a statement.

"The book is a danger to national security and peaceful and orderly society," he said, adding that anyone found breaching the ban imposed on Tuesday could face up to three years in jail and a fine of 60,000 baht ($1,800).

Thailand's revered King Bhumibol Adulyadej, 86, is protected by tough royal defamation rules under which anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

Marshall's book, which explores Thai politics including the taboo issue of royal succession, has been banned under the printing act rather than lese majeste legislation -- seemingly widening the legal net to prevent debate about the monarchy.

Critics say the royal defamation law, as well as the later Computer Crimes Act, stifle free speech and are used to silence political opponents.

Since grabbing power in May, junta leader and now premier Prayut Chan-O-Cha has repeatedly vowed to crack down on anyone who insults the monarchy, which attracts deep loyalty among the military and royalist Bangkok-based establishment.

Some experts believe that a struggle is unfolding to decide who will run the country when the more than six-decade reign of the revered but ailing king, currently in hospital after a gallbladder operation, eventually ends.

Discussion on succession is restricted in Thailand under the lese majeste law.

In September, rights group Amnesty International said there had been an "unprecedented" number of people charged with insulting the monarchy since the coup, echoing concerns raised by the United Nations.

Last week a 24-year-old student was jailed for two-and-a-half years for defaming Thailand's monarchy,
after pleading guilty to posting an insulting Facebook message under a pseudonym.

In another recent case, a 67-year-old man was charged with insulting the monarchy by scribbling anti-royal comments on the wall of a public toilet.

Thai authorities have previously banned other books deemed critical of the royals including an unauthorised biography of the king, "The King Never Smiles", by Paul Handley.

Marshall was not immediately available for comment. But in an earlier post on Twitter he said: "Many thanks to the Royal Thai Police for banning my new @ZedBooks publication 'A Kingdom in Crisis'. Great for publicity!"