Where did the Sword in the Stone come from?

According to Britannia.com, the story of the Sword in the Stone is referenced only in Merlin's history. Following what is most popularly believed, that Merlin became the young boy's tutor, while he grew up with his foster-father, Sir Ector. Merlin arranged for the sword-in-the-stone contest by which he became king, creating the defining moment of Arthur's career.  Later, the magician met the mystic Lady of the Lake at the Fountain of Barenton and persuaded her to present the King with the magical sword, Excalibur, it is noted that this is not the sword in the stone. The only notable reference of the Sword in the Stone, is also found in Britannia, and it is in the history of London during the Dark Ages that Arthur's pulling of the sword in St. Pauls' churchyard, the scene which is referenced in the Disney movie, that is said to be "merely apocryphal". As is most of the history of King Arthur. While more is known about Merlin, and through Merlin it is mostly thought to be the means of Arthur's birth, through the deception of Uther leading to the conception of Arthur. This is why I choose to blog about them in the first place, I couldn't have one without the other, without Merlin there would be no Arthur, and without Arthur, how would Merlin defeat the Saxons, the goal in which he was preordained?