LONDON (NYTimes) - The writings, etched in dark ink on a small scroll, tell the sordid story of four debauched aristocrats who lock themselves away in a castle to play out their wildest sexual fantasies, which run the gamut from orgies and animals to torture - including placing a firework up a bottom.

Even their author - the Marquis de Sade, the 18th-century French nobleman whose libidinous antics helped break sexual mores and inspired the word sadism - considered his work "the most impure tale ever written since the world began".

France considered de Sade such a notorious philanderer that he was jailed under royal orders in the late 1700s, including in Paris' Bastille prison for over a decade, just before it was stormed by revolutionaries. It was during this time that he wrote his salacious text.

But more than two centuries later, on Tuesday (Dec 19), the French government recognised his work, titled 120 Days Of Sodom, Or The School Of Libertinage, as a national treasure. The government decision came just a day before his work was about to be sold off at an auction in Paris, and means that the document, Sade's earliest work of fiction, may not be taken out of the country for at least 30 months.

During that time, the state is expected to shore up funds to purchase it at international rates, according to officials involved in the sale and the government decision. Officials also designated André Breton's Surrealist Manifesto as a national treasure.

De Sade's work, written on a scroll measuring 90cm long and just 10cm wide, "is a serious document of literature, of France's literary history", said Frederic Castaing, an expert on 18th-century manuscripts and a member of a commission that advises the government on what works should be designated as national treasures. "Without a doubt, it is a writing that challenges, that reaches into the depths of humanity, of the obscure," he said.

De Sade, he added, was one of France's most influential authors of the 18th century, alongside Voltaire and Diderot, and inspired the surrealist movement in the 20th century. The Ministry of Culture said that the manuscript was "remarkable", given the prison conditions in which it was written and its extraordinary journey through different hands. The ministry also pointed to the work's "sulfurous reputation" and its influence on a number of 20th-century French authors. The writing, it said, "is of great significance, as much as it is his first work as it is his most radical and most monumental".

The manuscript of 120 Days Of Sodom was expected to go for up to 6 million euros (S$9.6 million) on Wednesday, as part of a sale of historic manuscripts owned by Aristophil, a French investment firm whose founder was charged last year with operating one of the art world's biggest scams.

The firm went bankrupt in 2015 after buying more than 100,000 manuscripts. The entire collection is being liquidated in a process that could take years. Claude Aguttes, of the eponymous auction house who is handling the sale, said the French government had agreed to buy the works by de Sade and Breton "at international market rates".

He said 120 Days Of Sodom was the last known work by de Sade to be held in private hands. But that could change, he said, if the government fails to come up with the cash within the next 30 months, after which the manuscript's sale could open up once again to foreign buyers.

De Sade kept his manuscript hidden behind a rock in his cell in the Bastille but he was unable to smuggle it out with him when he was transferred to an asylum in 1789, a loss that caused him to weep "tears of blood", according to scholars. The document went through various hands, including French aristocrats, a German collector, and most recently a leading collector of erotica in Switzerland.

The work and de Sade's other writings were banned from publication in France and overseas until the early 20th century, when a limited circulation found an audience in surrealist circles and, surprisingly, the medical community. "It is the most extraordinarily shocking thing ever written," said Will McMorran, a lecturer at Queen Mary University of London who translated 120 Days Of Sodom into English in 2016, the first time in decades.