CHI replaces EHS 4D, the incumbent on the business for a number of years, and won the business without a pitch.
The agency already has a place on the RBS Group roster and was appointed by Peter Deane, the head of marketing for Churchill, Green Flag and Privilege.
CHI has handled Green Flag's advertising since January this year.
WCRS is currently the incumbent on Churchill's ad business. The agency's relationship with the insurance brand is unaffected by CHI's recent appointment.
In recent years, WCRS has been responsible for producing a series of ads for Churchill featuring a range of celebrities, including the model Melanie Sykes, the boxer Ricky Hatton, the TV presenter Roy Walker and Rolf Harris.
The agency's most recent campaign starred the TV botanist David Bellamy, who was shown asking the brand's dog mascot to look after his plants while he was on holiday.
Churchill has ramped up its marketing activity in the past few months, and recently struck a deal with the holiday-park company Pontin's. As part of the deal, the nodding dog character will feature in its own "bluecoat" show at Pontin's resorts.