Formerly, it was thought that the Arthurian legend was the work of several inventive poets and romancers of the Middle Ages. The generally accepted theory now is that Arthurian legend developed out of stories of Celtic mythology. The most archaic form in which these occur in British sources is the Welsh Mabinogion, but much of Irish mythology is palpably identical with Arthurian romance.
It is probable that traditional Irish hero stories fused in Britain with those of the Welsh, the Cornish, and the Celts of North Britain. The resultant legend with its hero, Arthur, was transmitted to their Breton cousins on the Continent probably by the year 1000. The Bretons, famous as wandering minstrels, followed Norman armies over Western Europe and used the legend's stories for their repertory. By 1100, therefore, Arthurian stories were well known even in Italy.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.