According to Geoffrey of Monmouth, king of Britain in the sixth or fifth century BC.
In Chronicles of England, Scotland and Ireland we can read the following at the beginning of the eighth chapter:
Of Gorbodug and his two sonnes Ferrex and Porrex, one brother killeth another, the mother slaieth hir sonne, and how Britaine by ciuill warres (for lacke of issue legitimate to the gouernment) of a, monarchie became a pentarchie: the end of Brutus line.
This is followed by:
Gorbodug the sonne of Kinimacus began his reigne ouer the Britains [...] This Gorbodug by most likelihood to bring histories to accord, should reigne about the tearme of 62 years, and then departing this world, was buried at London, leauing after him two sonnes Ferrex and Porrex, or after some writers, Ferreus and Porreus.
In the margin it is written that Gorbodug was the eighteenth ruler over Britain.
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Chronicles of England, Scotland and Ireland | Raphael Holinshed, 1807