Queen Cordelia was a legendary queen of the Britons as accounted by Geoffrey of Monmouth. She was the youngest daughter of Leir and the second ruling queen of Britain.

Cordelia was Leir's favorite daughter, being the younger sister to Goneril and Regan. When Leir decided to divide his kingdom between his daughters and their husbands, Cordelia refused to flatter him. In response, Leir refused her any land in Britain or the blessing of any husband. Regardless, Aganippus, the king of the Franks, courted her and Leir granted the marriage but denied him any dowry. She moved to Gaul and lived there for many years.

Leir was eventually exiled from Britain and fled to Cordelia in Gaul, seeking a restoration of his throne which had been seized by his other daughters' husbands. She raised an army and invaded Britain, defeating the ruling dukes and restoring Leir. After Leir's death three years later, her husband Aganippus died and Cordelia returned to Britain and was crowned queen.

Cordelia ruled peacefully for five years until her sisters' sons came of age. They were the dukes of Cornwall and Albany, Marganus and Cunedagius and they despised the rule of a woman when they claimed proper descent to rule. They raised armies and fought against Cordelia, who fought in person at numerous battles. She was eventually captured and imprisoned by her nephews. In her grief, she committed suicide. Cunedagius succeeded her in the kingship of Britain in the lands southwest of the Humber. Marganus ruled the region northeast of the Humber. Civil war broke out between them soon after.