The Vatican has for the first time published the prayer the Knights Templar composed when "unjustly imprisoned", in which they appealed to the Virgin Mary to persuade "our enemies" to abandon "calumnies and lies" and revert to "truth and charity".

L'Osservatore Romano, the Vatican newspaper, said the prayer was further proof that the order, which was dissolved in the fourteenth century, was not heretical. The knights were innocent of the charges against them, which included the accusation that they worshipped idols such as a "monstrous statue, half man and half goat".

The L'Osservatore Romano article, by Barbara Frale, the Vatican Secret Archives scholar who has made a special study of the knights, said it was untrue that the knights were guilty of "decadence, heresy and immoral practices".

The move follows legal action by the alleged heirs of Knights Templar to force the Vatican to restore the reputation of the disgraced order and acknowledge that assets worth some 80 million pounds were confiscated.

The Association of the Sovereign Order of the Temple of Christ, based in Spain, says that when the order was dissolved by Pope Clement V in 1307, over 9000 properties, farms and and commercial ventures belonging to knights were seized by the Church.

It is not however demanding that they be handed back, only that the order be "rehabilitated". A British branch also claiming descent from the Knights Templar and based in Hertfordshire has called for a papal apology for the persecution of the order.

The Knights Templar were founded at the time of the First Crusade in the eleventh century to protect Christians making the pilgrimage to Jerusalem. The Order was endorsed by the Vatican, but when Acre fell in 1291 and the Crusaders lost their hold on the Holy Land support faded, amid growing envy of the order's fortune in property and banking, and rumours about its corrupt and arcane secret ceremonies.

Whispers said novices had to deny Christ three times, spit on the cross, strip naked and kiss their superior on the buttocks, navel, and lips and submit to sodomy. King Philip IV of France, who coveted the order's wealth, arrested its leaders and put pressure on Clement V to dissolve it. Several knights, including the Grand Master, Jacques de Molay, were burned at the stake.

However Dr Frale said the arrests by Philip IV were "absolutely illegal". She said "legends and inventions" about the knights had been perpetuated by "fantasies" such as Dan Brown's The Da Vinci Code and - "though of a different literary calibre" - Umberto Eco's Foucault's Pendulum.

She said that in 1312 Pope Clement had declared that the Templars were not heretics, and had explained that he had only dissolved the order to prevent Philip IV from "opening up a schism in the Church". She said proof of the Templars' fidelity to the Pope and Christian dogma lay in the prayer they had composed during their long imprisonment.

It was "beautiful and moving" and "full of poetry", Dr Frale said, but "incredibly has never been studied".The prayer is addressed to "Holy Mary, mother of God", the "consolation of those who hope", and "humbly implores" her to obtain freedom for the order "through the intercession of the angels, archangels, prophets, evangelists, apostles, martyrs, confessors and virgins". It adds that the Virgin Mary knows that "our enemies" have spread "calumnies and lies" about the order, and pleads with her to make them "return to truth and charity".

Last October the Vatican launched a scheme to market limited reproductions of Processus Contra Templarios (Papal Inquiry into the Trial of the Templars), also known as the Chinon document, which Dr Frale discovered in the Vatican archives in 2003 after realising that it had been wrongly catalogued.

She said the parchment proved Clement V had accepted the Templars were not guilty of heresy. Their initiation ceremony involved spitting on the Cross, but this was to brace them for having to do so if captured by Muslim forces.