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In page History of Wales:

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At the Battle of Chester in 616, the forces of Powys and other British kingdoms were defeated by the Northumbrians under Æthelfrith, with king Selyf ap Cynan among the dead.[5] It has been suggested that this battle finally severed the land connection between Wales and the kingdoms of the Hen Ogledd ("Old North"), the Brittonic-speaking regions of what is now southern Scotland and northern England, including Rheged, Strathclyde, Elmet and Gododdin, where Old Welsh was also spoken.[citation needed] From the 8th century on, Wales was by far the largest of the three remnant Brittonic areas in Britain, the other two being the Hen Ogledd and Cornwall.