When Richard II and John of Gaunt decided (in view of the latter’s rather belated marriage to Katherine Swynford) that the Beauforts should be legitimated, they did two things. First they obtained a dispensation from the Pope removing any impediments to the Gaunt-Swynford marriage and legitimating the Beauforts. For example, the fact that Gaunt had served as a godparent to one of Katherine’s Swynford children had to be dispensed as this was a bar in canon law to the marriage. Once these hurdles were overcome and the couple married, as far as canon law was concerned their children were legitimate.
So why did Richard II go on to pass a Parliamentary statute legitimising the Beauforts? After all, under canon law they were already legitimate! The answer is that In England that was not enough. They had to be legitimate under common law as well, and common law stated that children born before marriage were illegitimate. The Statute of Merton (1235) had reaffirmed this common law position despite the opposition of the clergy.
Richard II and Gaunt were busy men, and didn’t have statutes passed for the fun of it. They were well aware that, without statutory underpinning, the Beauforts would not be eligible to inherit land. So the statute was passed. Yet another example of how, in English law relating to inheritance, the common and statute law of the land trumped canon law every time.
Some years later, John Beaufort, Earl of Somerset, asked for an exemplification of the legal position from his half-brother, Henry IV. He was possibly concerned about the impact of Henry’s usurpation on the statute law of Richard II, especially given that he, Somerset, had suffered some pains and penalties at the time, including his loss of the marquisate of Dorset.
Henry obliged, by producing Letters Patent which confirmed the status quo with the addition of a provision that this did not apply to inheritance of the crown itself.
Henry’s legal right to do this has been questioned. Certainly, no modern Government would ask the Queen to modify a statute by issuing Letters Patent, and if the Queen did so it would unquestionably be illegal. However, in the middle ages (and long after) the Crown had the right to vary or ‘dispense’ statutes if it saw fit. King James II was still doing this in the late 17th Century, and it was only the Glorious Revolution that led to the practice being made unequivocally illegal.
So Henry IV probably felt that he had the right to exclude the Beauforts, and it is at least arguable that under the law as it was understood at that time he was entitled to do so. It would, after all, have been straightforward enough to put a revised statute through Parliament had he deemed it necessary.