John of Gaunt, Edward III's third son, married his mistress Katherine Swynford on the 13th January 1396. Pope Boniface IX issued a Bull in September of that year, acknowledging the marriage and legitimising their four children, each surnamed Beaufort. Richard II further issued letters patent which was confirmed by parliament in February 1397.
Translation taken from “Excerpta historica: or, Illustrations of English history”, Ed. Samuel Bentley, ps.153-154.
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“Be it remembered, that on Tuesday the fifteenth day of Parliament, the Chancellor, by the King's command, declared how our Holy Father the Pope had, in reverence of the most excellent person of the King, and of his honorable uncle the Duke of Guyenne and of Lancaster, and of his blood, enabled [habliez] and legitimatized Sir John de Beaufort, his brothers and his sister, and that therefore our Lord the King as entire Sovereign [entier Emperor] of this Realm of England, for the honor of his blood, wills, and hath of his full royal power enebled and of his own proper authority made the said John, his said brothers and sister, muliers,1) and also pronounced and published the ability [l'abilite] and legitimatization according to the form of the King's charter thereof made. The which charter was read in full Parliament and delivered to the said Duke, father of the said John, and his said brothers and sister, the tenor of which charter ensueth. Richard by the grace of God, King of England and France, and the Lord of Ireland, to our most dear cousins the noble men, John the Knight, Henry the Clerk, Thomas 'Domicello,' and to our beloved the noble woman Joan Beauford 'Domicelle,' the most dear relatives of our uncle the noble John Duke of Lancaster, born our lieges, greeting, and the favour of our royal majesty. Whilst internally considering how incessantly and with what honours we are graced by the very useful and sincere affection of our aforesaid uncle, and by the wisdom of his counsel, we think it proper and fit that, for the sake of his merits, and in contemplation of his favors, we should enrich you (who are endowed by nature with great probity and honesty of life and behaviour, and are begotten of royal blood, and by the devine gift are adorned with many virtues,) with the strength of our royal prerogative of favour and grace. Hence it is, that, yielding to the entreaties of our said uncle your father, we do, in the fullness of our royal power and by the assent of Parliament, by the tenor of these present empower you, who as it is asserted suffer from the want of birthright, (notwithstanding such defect, which, and the qualities thereof, we take to be in these presents sufficiently expressed) to be raised, promoted, elected, assume, and be admitted to all honours, dignities, [inserted here by Henry IV: except to the royal dignity] pre-eminencies, estates, degrees and offices public and private whatsoever, as well perpetual as temporal, and feodal and noble, by whatsoever names they may be designated, whether they be Duchies, Principalities, Earldoms, Baronies or other fees, and whether they depend or are holden of us mediately or immediately, and to receive, retain, bear, and exercise the same as freely and lawfully as if ye were born in lawful matrimony, and you and every of you do restore and legitimatize: any statues or customs of our realm of England to the contrary thereof made or observed (which we consider to be herein fully expressed) in anywise notwithstanding. Witnessed by the King at Westminster the 9th2) day of February.”
From “The King's Mother: Lady Margaret Beaufort, Countess of Richmond and Derby”, p.24. Michael K.Jones, Malcolm G.Underwood. Available from Amazon.co.uk Amazon.com - links are non-affiliated
“Their legitimation in 1397 had been unconditional. Yet the enabling act referred to property, office and noble rank. Although it accorded the right to dignities it conferred no royal interest or right and title to the throne. Here the language of the act was significant. It used no words of empire, majesty or sovereignty, such as 'regnum', 'summa potestas', 'corona' or 'maiestas' that would enable a man to be an heir of a kingdom.”
John Beaufort requested an exemplification on the 10th February 1407 and, to the original wording, Henry IV added excepta dignitate regali (beside regal), therefore barring any issue of the Beaufort line from the throne. There was, however, no approval from parliament, with the original act unamended.