| 3 Alarms Limited | Construction | 2005 |
| A C F Flooring Limited | Wholesale & retail | 2007 |
| A Patterson Installation Limited | Construction | 2006 |
| A T H Solutions (UK) Limited | Information & communication | 2002 |
| A.Hedley (Motor Engineers) Limited | Wholesale & retail | 1975 |
| A.L.M. Products Holdings Limited | Financial services | 2018 |
| A.T. Taxis Limited | Transport & logistics | 2003 |
| Acorn Alarms Ltd | Manufacturing | 2007 |
| Adimo Homes Ltd | Real estate | 2026 |
| Advantg Limited | Wholesale & retail | 2025 |
| Aeris Holding Limited | Financial services | 2025 |
| AHS (Site Services) Ltd | Construction | 2025 |
| Air Handling Systems Limited | Manufacturing | 2000 |
| Airtek Safety Netting Services Limited | Construction | 2007 |
| AKL Utilities Limited | Construction | 2005 |
| ALI Ahmed Limited | Wholesale & retail | 2025 |
| Allan Oliver Reinforcing Limited | Construction | 1999 |
| Allan Whiteford Ltd | Professional & scientific | 2010 |
| Allington Commercial Ltd | Real estate | 2026 |
| Allroundthehouse Limited | Information & communication | 2010 |
| Allure Developments (Darlington) Limited | Construction | 2018 |
| Allure Developments Limited | Manufacturing | 1990 |
| Allure Trading Limited | Manufacturing | 2022 |
| Almond Windows Limited | Construction | 2003 |
| AMB Group Ltd | Construction | 2016 |
| Ambient Creative Services Ltd | Professional & scientific | 2000 |
| Andrew Douglass (Printers) Limited | Manufacturing | 1973 |
| Angus Ward Holdings Limited | Water & waste | 2025 |
| Annthony Limited | Wholesale & retail | 2025 |
| Apex Tooling & Moulding Solutions Limited | Manufacturing | 2025 |
| Applied Ecological Services Limited | Professional & scientific | 1996 |
| APS Services (North East) Limited | Professional & scientific | 1996 |
| Arcadia Leisure Limited | Administrative & support | 1996 |
| Area I.T. Consultants Limited | Information & communication | 2005 |
| Ashton Imports Limited | Real estate | 2000 |
| Atlas Washroom Holdings Ltd | Financial services | 2018 |
| Auckland Business Consultancy Ltd | Professional & scientific | 2026 |
| Authentic Conservatories Ltd | Construction | 2004 |
| AWT Business Solutions Limited | Professional & scientific | 2005 |
| Aycliffe Property Development Limited | Real estate | 2004 |
| B Snowdon Motors Limited | Wholesale & retail | 2005 |
| B. & a. Design & Manufacture Limited | Manufacturing | 2002 |
| Barnard Castle Cricket Club Limited | Arts & recreation | 2016 |
| Baseline Associates Limited | Administrative & support | 2000 |
| Bathroom World Darlington Limited | Wholesale & retail | 2004 |
| Battery Power Technology Ltd | Manufacturing | 2003 |
| BC Salon Limited | Other services | 2017 |
| Beamish Residential Care Home Limited | Health & social care | 2001 |
| Bescol Limited | Wholesale & retail | 1984 |
| Betconnect Client Services Limited | Information & communication | 2020 |
| Betconnect Ltd | Information & communication | 2011 |
| Beverley Henry Limited | Wholesale & retail | 2025 |
| Bezo Properties Ltd | Real estate | 2021 |
| Birch Grove Preservation Limited | Extraterritorial | 2006 |
| Birtley Osteopathic Clinic Limited | Health & social care | 2003 |
| Black on Black Ltd | Other services | 2002 |
| Boam Automotive Engineering Limited | Wholesale & retail | 2014 |
| Brancepeth Engineering Limited | Manufacturing | 2005 |
| Bridge END (North East) Limited | Hospitality | 2000 |
| Bridge House Estates Ltd | Real estate | 2020 |
| Brilliantworks Limited | Professional & scientific | 2019 |
| Brindle Construction Limited | Construction | 1999 |
| British - Israel Trust Company Limited(The) | Real estate | 1938 |
| Broad Studios Ltd | Administrative & support | 2022 |
| Brotherton and SON (Bishop Auckland) Limited | Wholesale & retail | 1964 |
| Browns Rowing Boats Limited | Transport & logistics | 2000 |
| Burleigh Technical Services Ltd | Professional & scientific | 2018 |
| Business & Property Solutions Limited | Real estate | 2004 |
| Bussey & Armstrong Projects Limited | Construction | 1999 |
| Butler & GEE Management Ltd. | Professional & scientific | 1998 |
| Bychloelew Ltd | Other services | 2025 |
| C H 18 Ltd | Hospitality | 2011 |
| C X 980 Ltd | Hospitality | 2021 |
| C.A. Group Limited | Construction | 1983 |
| C.T. Steel Site Services Limited | Construction | 2001 |
| Cable and Support Management Limited | Professional & scientific | 2004 |
| Caitlin Riley Limited | Wholesale & retail | 2025 |
| Cakes 'N' Bakes Cafe Ltd | Hospitality | 2023 |
| Calumn Robert Limited | Wholesale & retail | 2025 |
| Canny Lads Media Holdco Limited | Extraterritorial | 2021 |
| Canny Lads Media Limited | Extraterritorial | 2016 |
| Caravel Global Limited | Transport & logistics | 1998 |
| Caravel Liner Agencies (Darlington) Limited | Transport & logistics | 1990 |
| Carlton Bonds PLC | Financial services | 2019 |
| Carrick Contractors Limited | Construction | 2002 |
| Carver Commercial Ltd | Real estate | 2003 |
| Castle Estates (Brancepeth) Limited | Real estate | 1961 |
| Casual Terrace Wear UK Ltd | Extraterritorial | 2020 |
| Centaur Training & Development Limited | Administrative & support | 2011 |
| Chemian Technology Limited | Manufacturing | 1995 |
| Chevron Industrial Roofing Limited | Wholesale & retail | 1979 |
| Cifo Limited | Financial services | 2024 |
| City Carpets (Durham) Limited | Wholesale & retail | 2004 |
| Cleves Property Maintenance Limited | Administrative & support | 2002 |
| Clive Owen LLP | | 2005 |
| Coates & Co. Financial Planning Limited | Professional & scientific | 2005 |
| Colton Excavations Limited | Construction | 1999 |
| Colton Group Holdings Limited | Professional & scientific | 2023 |
| Colton Group Limited | Professional & scientific | 2007 |
| CON Mech Durham 1 Limited | Financial services | 1989 |