Charles I may not have been a brilliant king but he has come to us as a devoted husband, a connoisseur of art who cleaned up the drunkenness and profanity tolerated by his father. He didn't lose his head over women.

Drunkenness and profanity returned with a vengeance with his son Charles II, who spent more time with Nell Gwyn and a series of mistresses than his royal spouse.

So when Sarah Poynting, of Keele University, announced she had cracked two letters written in code that revealed a bawdy, lustful side of Charles I - proposing a "swiving" to a courtier's stepdaughter - her colleagues were incredulous. Surely she meant Charles II?

While preparing a scholarly edition of the king's writings, the historian examined the letters Charles I smuggled out on scraps of paper from Carisbrooke Castle, where he was imprisoned in 1648, the year before his execution. Two were to Jane Whorwood, stepdaughter of one of the king's former courtiers; she was a tall, red-haired woman with a pock-marked face.

Previous historians have assumed that while he formed a strong bond with her that may have been tinged with mildly romantic feelings, the king's moral rectitude and famous love for his wife made a sexual liaison very improbable. Charles's letters were partly in cipher, and Dr Poynting has shown that an earlier historian decoded a key word incorrectly.

Charles wrote that Jane could easily visit him, but warned that they would not be able to speak privately without special permission. His letter then switched into cipher.

According to earlier attempts to decode the letter, the next sentence read: "Yet I imagine that there is one way possible that you may get answering from me", followed (not in cipher) by "you must excuse my plain expressions". But Dr Poynting realised that "answering" could only be correct if the king had made three separate mistakes in the cipher for one word. When she worked through the passage, a rather different meaning emerged: "I imagine that there is one way possible that you may get a swiving from me". He then described how she could have secret access to his rooms.

In the 17th century, "swiving" was a wholly obscene word for sex, found most commonly in the pornographic verses of the Earl of Rochester, who used it to describe the notorious sexual activity of Charles's son after the Restoration. It is hardly surprising to find the word in association with Charles II, but to meet it coming from the pen of his father, who, according to the royal servant the Earl of Clarendon, "could never endure any light or profane word", is astonishing, especially as he is known to have been engaged in serious reading and prayer at the time.

Dr Poynting said: "The king is known to have been devoted to his wife, Henrietta Maria, and remembered for refusing to tolerate the drunkenness and immorality that marked the court of his father, James VI and I. Charles's sexual probity has been so taken for granted that when I first told colleagues about my discovery, they didn't believe I could be right until I went step-by-step through the cipher. There is no reason, though, to find such apparent contradictions incredible; it simply means that we have begun to understand a little more about a complex man at one critical time. Charles may have been a unique captive, but there were moments when he shared the concerns of many other prisoners: self-justification, escape, and sex."