thumb|351x351px|Map of Roman Britain and the client kingdom territories in 150 AD.
The Roman client kingdoms in Britain were native British polities that maintained allied relationships with the Roman Empire and retained local rulers under Roman patronage. Alternatively, the Romans created or enlisted some client kingdoms when influence without direct rule was desirable. Client kingdoms were ruled by client kings. In Latin these kings were referred to as rex sociusque et amicus, which translates to "king, ally, and friend". The relationships between client kingdoms and Rome depended on the circumstances in each kingdom. The system developed further during the following century, particularly under Augustus's influence, so that by the time of the Roman invasion in AD 43 several Roman client kingdoms had become established in the south of Britain. Client kingdoms were annexed when Rome reaffirmed its power in Britain or when client rulers were no longer able to govern their territories effectively.
These were also partially due to the expansion of the Catuvellauni under Cunobelinus in the southeast, and partly as a result of the invasion itself, and included Cogidubnus of the Regni, Prasutagus of the Iceni and Cartimandua of the Brigantes and, probably, Boduocus of the Dobunni. The antecedents of the Regni, the Atrebates, had (in their Gallic and British forms) been client kingdoms of Rome since Caesar's first invasion in 55 BC. In the north of Britain, ongoing border struggles across the defensive walls led to the establishment of buffer states, including the Votadini in Northumberland. In 54 BC the invasion was considered a success but in Caesar's eyes the island yielded little reward and he left without leaving a garrison to watch over his latest conquest.
Client kingdoms were initially established as auxiliary support for the Roman army under Claudius in the expanding empire, but were reimagined as the way for Rome to achieve the power status of a unified empire.
Client kingdoms
Client kingdoms of Southern Britain
Trinovantes and Catuvellauni
Client status: 54 BC -c.39 AD<br />
Location: lands in present-day south-East England
In 54 BC, Julius Caesar set up Mandubracius of the Trinovantes as a client king and established the Catuvellauni as a tributary state of Rome. This led to his defeat in 50 AD and search of refuge with the Brigantes, and ultimately a betrayal by Cartimandua in 51 AD.
Atrebates, later Regni or Regnenses
Client status: 55 BC-70s AD<br />
Location: Roughly modern-day Hampshire and West Sussex (capitals now Silchester and Chichester)
The Belgic Atrebates were led by a semi independent client king, Commius, in Gaul when Caesar left Britain after his first invasion. When Commius began to conspire against Rome, he was forced to flee to Britain in 54 BC. He named himself king of his people and ruled until approximately 20 BC. Commius was succeeded by three of his sons. First, Tincomarus, from 25/20 BC to 7/8 AD. He was more sympathetic to Rome than his father had been, and based on numismatic evidence styled himself rex, implying client kingship status under the Empire. He was expelled in 7/8 AD, seeking refuge with the Romans.
Client kingdoms of Northern Britain
Brigantes
Client status: c. 46 AD-
Location: Pennines of South Yorkshire to north of the Tyne.
The Brigantes were given client status but were not considered a conquered Celtic tribe, despite accepting the Romans as the governing power.
Votadini
Client status: c.140s-c.410 AD<br />
Location: Present-day southeast Scotland and present-day northeast England, including modern-day Northumberland
The Votadini were a Brythonic people who lived under the direct rule of Rome between Hadrian's Wall and the Antonine Wall from 138 to 162 AD. When the Romans withdrew behind Hadrian's Wall in 164 AD, they left the Votadini as a client kingdom, a buffer zone against the Picts in the north.