Doctors were summoned and at first he rallied, but over the next two days the King's condition deteriorated. By Thursday, he was plainly dying. As doctors abandoned their efforts, the clergy took their place. These were Anglican churchmen, the Archbishop of Canterbury and several bishops; Charles was a Protestant, and head of the Church of England. They urged him to receive the last sacrament, but he would have none of it, insisting there was plenty of time.
What happened next has been described by different witnesses in slightly different terms, but a few facts are beyond dispute. The King's brother, the Duke of York (soon to be King James II), surprised the the assembled churchmen and courtiers by announcing: 'Gentlemen, the King wishes everybody to retire except the Earls of Bath and Feversham.'
Once the room had been cleared, a secret door was opened and through it stepped John Huddleston, a Catholic priest who, 33 years before, had helped the young Charles escape from Cromwell's Roundheads. The priest asked Charles if he wished to be received into the Roman Catholic faith and he said that he did.
Charles took Communion, was anointed with holy oil and spent some time in prayer. Huddleston then left by the secret door and the courtiers were allowed back. The next morning the King was dead.
By the time the truth emerged of what had transpired in those 45 secret minutes, it was already overshadowed by the crisis associated with succession to the throne of James II, whose Catholicism was no secret. But Charles II's deathbed conversion must nevertheless rank among the most extraordinary events in the history of the British monarchy.
Last week, when it was announced that the Duchess of Kent, wife to the Queen's cousin, was converting to Catholicism, there was much brouhaha. Imagine for a moment the effect of the conversion of the Queen herself. Now transfer that back to 1685, a time when Catholicism was proscribed by law, priests were often executed and the country was in a perpetual frenzy about Popish plots.
Why did Charles do it? Certainly, he tolerated Catholics at court and throughout his life many had been close to him, including his mother, his wife, his brother and a number of his mistresses. But he had always been deeply conscious of his role as the head of the Church of England, from which much of his authority stemmed. What is more, in his eyes his father, Charles I, had died for that Church. There is nothing in the contemporary accounts, however, to suggest that Charles did not make the decision of his own free will.
Whose idea it was we do not know, as several people later claimed the credit. James, it seems, had discussed the idea with the Queen early in the week, but they rejected it as impossible. The French ambassador wrote to King Louis XIV in Paris soon afterwards to say that he himself had taken the initiative, while a Catholic priest by the name of Benedict Gibbon also published a tract saying that the suggestion had come from him.
Antonia Fraser, a biographer of Charles (and herself a Catholic), identifies as the prime mover the King's favourite mistress in his later years, Louise de Keroualle, Duchess of Portsmouth. By this account, Louise put her own interpretation on Charles's reluctance to accept the Anglican rites, and declared that 'at the bottom of his heart the King is a Catholic'. The message was passed to James, who was stirred to call for Huddleston.
Another biographer, Ronald Hutton of Bristol University (who describes himself as a pagan), views the affair with a sceptical eye. 'What we can never know,' Hutton writes, 'is the spirit in which he embraced Catholicism. Was his action the triumphant climax to years in which he had longed for such a moment but dared not, for political reasons, bring it to pass? Or was the King indifferent to forms of religion and, so far gone that he could barely comprehend what was being asked, humouring his brother?'
Whichever was the case, Charles's last conversations were not about his immortal soul. Uppermost in the thoughts of the Merry Monarch (father of 12 bastards by seven different women), was the future care of his surviving mistresses, Louise de Keroualle and Nell Gwyn. The words 'Let not poor Nelly starve' may not be strictly faithful to fact, but the sentiment is.