| Commonwealth Group Services Ltd | Financial services | 2025 |
| Communities Together Durham | Health & social care | 2014 |
| Community of the Well Ltd | Other services | 2021 |
| Company Doctor Direct Ltd | Professional & scientific | 2006 |
| Company Light Ltd | Transport & logistics | 2025 |
| Compass Environmental Services Limited | Water & waste | 2014 |
| Complement Genomics Limited | Professional & scientific | 2000 |
| Complete Cleaning Solutions NE Limited | Other services | 2025 |
| Complete FM Solutions Ltd | Administrative & support | 2018 |
| Complete Ground Care Ltd | Administrative & support | 2025 |
| Complete Quality Solutions Ltd | Professional & scientific | 2023 |
| Complete Utility & Infrastructure Ltd | Construction | 2025 |
| Compressed Air Specialist's Teesside Ltd | Other services | 2024 |
| Compro Limited | Professional & scientific | 2016 |
| Computer Technology Services (Durham) Ltd | Information & communication | 2010 |
| Concept 2 Leisure Limited | Hospitality | 2019 |
| Concept Investment Management Ltd | Professional & scientific | 2012 |
| Concept Media Academy Ltd | Administrative & support | 2021 |
| Concurrent Ltd | Information & communication | 2006 |
| Conduit Construction Network Limited | Manufacturing | 2005 |
| Conelab Group Limited | Financial services | 2025 |
| Coniscliffe Court Residents Association Limited | Households | 1982 |
| Conservation Ecology Consultancy Limited | Professional & scientific | 2024 |
| Consett Innovation Company Limited | Financial services | 2019 |
| Consett Print Services Ltd. | Manufacturing | 2017 |
| Consortium Wealth Management Limited | Financial services | 2015 |
| Container Storage UK Ltd | Transport & logistics | 2019 |
| Conteh Properties Limited | Real estate | 2024 |
| Convenience Revolution Limited | Wholesale & retail | 2025 |
| Convergent Labs Ltd | Manufacturing | 2025 |
| Convoy Sync Ltd | Information & communication | 2025 |
| Cook and Rippon Plasterers Limited | Construction | 2025 |
| Cook Chill Durham Ltd | Hospitality | 2023 |
| Cook's Barrage Limited | Hospitality | 2022 |
| Cook's Cafe Group Limited | Hospitality | 2022 |
| Cook's Forum Limited | Hospitality | 2022 |
| Cook's Splash Limited | Hospitality | 2022 |
| Cooks’ at IB Limited | Hospitality | 2026 |
| Cooper & Mcgurk Limited | Professional & scientific | 2018 |
| Cooper Properties NE Ltd | Real estate | 2022 |
| Cooper Stott Solicitors Ltd | Professional & scientific | 2014 |
| Cooper Systems Design Limited | Manufacturing | 2025 |
| Coopers Close Management Limited | Households | 1986 |
| Copa International Consulting Ltd | Professional & scientific | 2022 |
| Coptek Electrical Ltd | Construction | 2021 |
| Corbett & Saint Ltd | Hospitality | 2024 |
| Core Coaching & Careers Limited | Education | 2026 |
| Core Contract Furniture Ltd | Manufacturing | 2023 |
| Core Eco Limited | Wholesale & retail | 2021 |
| Core Imaging Partners Ltd | Professional & scientific | 2026 |
| Core Installations Limited | Construction | 2022 |
| Core Logistics UK Limited | Manufacturing | 2013 |
| Corepeople Recruitment Limited | Administrative & support | 1998 |
| Coretech (Sports) Limited | Other services | 2012 |
| Corewebux Limited | Information & communication | 2024 |
| Coriopat Consulting Ltd | Professional & scientific | 2013 |
| Cornerspeed Limited | Professional & scientific | 2005 |
| Cornsay Valley Limited | Real estate | 2022 |
| Cortech F&S Holdings Ltd | Financial services | 2022 |
| Cortech Fire & Security Systems Limited | Administrative & support | 2000 |
| Coskun Ticaret Ltd | Wholesale & retail | 2025 |
| Costa Andaluza Limited | Hospitality | 2025 |
| Costa Arena Y SOL Limited | Hospitality | 2025 |
| Costspex Limited | Manufacturing | 2006 |
| Cottage Energy Limited | Households | 2014 |
| Coughtrey Construction Limited | Construction | 2018 |
| County Durham and Darlington Fire and Rescue Community Interest Company | Professional & scientific | 2013 |
| County Durham Building & Joinery Ltd | Construction | 2010 |
| County Durham Catering Limited | Hospitality | 2023 |
| County Durham Commercials Limited | Wholesale & retail | 2012 |
| County Durham Developments Ltd | Real estate | 2022 |
| County Durham Funeral Directors Ltd. | Other services | 2019 |
| County Durham Services Ltd | Construction | 2022 |
| County Group Solutions Limited | Information & communication | 2020 |
| Coward Pharmacy Limited | Wholesale & retail | 2000 |
| COX Bailey Medical Limited | Real estate | 2010 |
| Coxhoe Driving Services Limited | Transport & logistics | 2008 |
| Coxhoe Groundworks Durham Drainage Limited | Construction | 2010 |
| COY Consulting Services Ltd | Professional & scientific | 2018 |
| CP Medical Reports Limited | Health & social care | 2017 |
| Cpmt Holdings Limited | Financial services | 2023 |
| CPY Construction NE Limited | Construction | 2024 |
| CR Structural Services Limited | Information & communication | 2017 |
| Crab Civils Ltd | Construction | 2025 |
| Crabit Limited | Information & communication | 2021 |
| Crafted Products and Services Ltd | Manufacturing | 2022 |
| Crafty Apps Limited | Wholesale & retail | 2010 |
| Craig James Properties Ltd | Real estate | 2023 |
| Craig-Robinson Ltd | Construction | 2024 |
| Craigs Coaching Limited | Other services | 2012 |
| Cranex Limited | Manufacturing | 2000 |
| Cranfield Golf Limited | Education | 2017 |
| CRC Distribution Limited | Wholesale & retail | 2023 |
| Create North Ltd | Professional & scientific | 2002 |
| Creative & Distinctive Hair & Beauty Products Ltd | Wholesale & retail | 2012 |
| Creative Allsorts NE CIC | Arts & recreation | 2024 |
| Creative Property Sourcing Ltd | Real estate | 2018 |
| Creative Teaspoon Ltd | Professional & scientific | 2014 |
| Creativity Works North Community Interest Company | Education | 2023 |
| Crescent Homecare Limited | Health & social care | 2009 |