Was England ever on the verge of becoming an Islamic state? In 1215 King John was forced to accept the Magna Carta, that touchstone of English liberties. But according to one medieval chronicler, only two years previously he was toying with passing the country over to Sharia.

The claims appear in the Chronica Majora written some years after the event by a Benedictine monk by the name of Matthew Paris.

In 1207 Pope Innocent III placed England under an interdict that effectively closed down the country's churches. He excommunicated John two years later. Facing war with France and rebellion at home, the monarch was in a tight spot.

If Matthew Paris is to be believed, this was the background to perhaps the most bizarre diplomatic initiative in English history. John dispatched Thomas of Erdington, Radulus, son of Nicholas Esquire, and a cleric, Robert of London, on a top secret mission to Morocco.

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On arrival, they approached the powerful Almohad caliph, Muhammad an-Nâsir. Their task was to win his military assistance to help to see off John's converging enemies. Paris claimed that they brought a letter from the King offering to place England at the caliph's disposal and promising that John “would not merely relinquish the Christian faith, which he considered vain, but would adhere faithfully to the law of Muhammad”. Far from being impressed, the caliph sent John's emissaries away, curtly assuring them that he had no intention of allying with someone so lacking in faith that he was intent on becoming an apostate for the sake of political expediency. Thus rebuffed, John ended up having to appease the Pope and the barons instead.

Can this bizarre Moroccan adventure possibly be true? Although John was certainly exploring various extreme diplomatic options at the time, many leading medieval historians believe that the story of his offer to convert to Islam must be largely or wholly a work of fiction.

After all, while Paris may have been an early historian, he was also a propagandist, intent on misrepresenting the King's position. On the other hand, he claimed an impeccably placed source for the story. Paris was a monk at St Albans Abbey where the guardian was Robert of London, supposedly one of King John's Moroccan posse.

At any rate, if the whole story was a concoction, then it ended up missing its target. While subsequent generations of British historians have tended to pass over it as a red herring, it has gained wide currency across the Maghreb.

Only the discovery of John's letter of self-abasement lying undisturbed in some Moroccan archive could prove the story true. And if we could read it now, would apologists for his late Majesty be able to assert that his words had, in fact, been taken terribly out of context?

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