GEOAmey, a joint venture between US prison transportation firm GEO and British public service provider Amey last year, has brought in the agency following a six-way-pitch.
Last March the group was handed an initial seven-year contract worth £90m a year, with the possibility of a three-year extension, to provide prison custody services across England and Wales.
GEOAmey has charged Citigate with a lucrative brand-building campaign to grow its corporate presence in the UK as well as handling reactive media relations.
The agency will run a 24/7 press office function, stakeholder relations and social media service. Citigate will also handle strategic risk management and crisis comms.
The final contractual details were still being finalised at the time PRWeek went to press, but Citigate acknowledged that it had been selected via the competitive pitch process.
The team will include corporate team directors Jonathan Smith, Ewan Robertson, Ian Morris and Holly Rees. The agency is understood to be reporting to Gill Dalrymple, head of comms and marketing at GEOAmey.
The GeoAmey joint venture manages more than two-thirds of the UK's prison custody services, handling the entirety of prisoner transportation work in the UK and Wales with the exception of London and the eastern region.
The company conducts more than 2,000 prisoner transfers a day.
Citigate previously worked for global security firm G4S before the wide-ranging account moved to Bell Pottinger last year. GEOAmey replaced G4S when the company was handed its prisoner transportation contracts.