The book is unlikely to turn up in Easons or Waterstones: measuring 27 by 22in, printed on artificial parchment with replicas of the original papal seals, it is as close as the publishers can get to the appearance of the original document, which turned up in the Vatican's secret archives in 2001, having been mislaid for more than 300 years.

Academics and fans of Dan Brown's thrillers will be eager to get their hands on the book but it costs 5,900 per copy, and most of the 799 copies have already been reserved by specialist libraries. The 800th will be given to the Pope.

It ought to make uncomfortable reading for him. The Knights Templar, the order of monastic knights set up to defend Al-Aqsa Mosque in Jerusalem, seized during the First Crusade, went into decline after Christians were expelled from the Holy Land in the 13th century. King Philip IV "the Fair" of France owed the order large amounts of money and land; to avoid repaying the debt, he prevailed on Pope Clement V, based in Avignon and dependent on his good offices, to put members on trial for heresy.

The pope tried them, and while he found them guilty of immorality, the key charge of heresy was found to be false. It had been alleged that while in Jerusalem they had been in the custom of spitting on crosses, and underwent an initiation ceremony that involved kissing. They persuaded the pope and his judges that the spitting was done to prepare themselves for the dissembling they would be obliged to practice if captured by the Saracens, while the kissing was a way of promising complete obedience. The pope accepted their arguments and absolved them of heresy. Philip pressured the Pope to reverse his verdict, and the head of the order and his closest associates were burnt at the stake. Their riches were handed to a rival knightly order, and the surviving knights melted away.