The Civil War of Albany and Cornwall was a legendary war in Britain following the death of Leir and the acension of Queen Cordelia as recounted by Geoffrey of Monmouth.

Following a fairly peaceful five years of rule, Cunedagius, the Duke of Cornwall, and Marganus, the Duke of Albany, both grandchildren of King Leir, came of age and contested the authority of Queen Cordelia. They fought against Cordelia and finally imprisoned her. Soon after, she committed suicide in her despair.

To avoid conflict, they divided the kingdom into two sides, Cunedagius ruling the southwest from the Humber and Marganus ruling the northeast. Two years after they split the island, Marganus invaded Cornwall and destroyed much of the land. Cunedagius met him in battle and defeated him. Marganus fled throughout Britain until he was cornered in Wales. Cunedagius killed him there and became high king of all Britain, ending the civil war.