| MF Global Investments Limited | Financial services | 2025 |
| Mirvsys Limited | Transport & logistics | 2007 |
| Mitg Ltd | Administrative & support | 2005 |
| MJ & a Taylor Limited | Wholesale & retail | 2004 |
| Modern Office Limited | Wholesale & retail | 2011 |
| Moount Limited | Wholesale & retail | 2025 |
| Multi Task Cleaning Services Limited | Administrative & support | 2026 |
| My Green Power Limited | Construction | 2023 |
| Myah Limited | Wholesale & retail | 2026 |
| Nadoc Interior Ltd | Administrative & support | 2024 |
| Naomi Apparel Ltd | Wholesale & retail | 2025 |
| Neil Storey Plumbing & Heating Limited | Construction | 2004 |
| Nett Solutions Ltd | Real estate | 2020 |
| Nevern Projects Limited | Construction | 2005 |
| New Level Landscapes Limited | Administrative & support | 2007 |
| Newgate Investments Limited | Real estate | 1959 |
| NJB Investments Limited | Financial services | 2023 |
| Norco Heat Services Limited | Construction | 2006 |
| North East Excavations Limited | Real estate | 1970 |
| North East Fuel Group Ltd | Wholesale & retail | 2019 |
| North East Granite Company Limited | Manufacturing | 2002 |
| North East Groundworks Solutions Ltd | Construction | 2023 |
| Northern Breakers Limited | Wholesale & retail | 2000 |
| Northern Life Sciences Limited | Health & social care | 2010 |
| Northern Melts Limited | Wholesale & retail | 2006 |
| Northumbria Health Ltd | Manufacturing | 2025 |
| Northumbria Pharma Group Holdings Ltd | Financial services | 2025 |
| Northumbria Pharma Thrombosis Limited | Manufacturing | 2025 |
| Octobox Holdings Limited | Financial services | 2017 |
| OIL Tax Consultants Limited | Professional & scientific | 2007 |
| Olding Limited | Wholesale & retail | 2025 |
| Ollie & Erin Couriers Ltd | Transport & logistics | 2024 |
| Orchard House Services Limited | Professional & scientific | 1998 |
| Oswald House Care Home Limited | Health & social care | 2003 |
| Ozware Ltd | Wholesale & retail | 2020 |
| P Barton Dairyman Limited | Wholesale & retail | 2002 |
| P.F. Trading (N.E.) Limited | Real estate | 2003 |
| Peking Villa Takeaway Ltd | Hospitality | 2022 |
| Pentam Limited | Professional & scientific | 2005 |
| Perk Properties Limited | Real estate | 2025 |
| Peter Clark Property Services Limited | Real estate | 2001 |
| Peter Redpath Flooring Limited | Construction | 2003 |
| PHP Holdings Limited | Professional & scientific | 2021 |
| Pinnacle Property & Development Limited | Real estate | 2003 |
| PJA Motors Limited | Manufacturing | 2007 |
| Playday Nursery Ltd | Education | 2002 |
| Pnts Holdings Limited | Financial services | 2022 |
| Power Tech (Northern) Limited | Construction | 2009 |
| Premier Fluid Solutions Limited | Construction | 2005 |
| Preseel Limited | Wholesale & retail | 2025 |
| Priestgate Morley Limited | Administrative & support | 2007 |
| Project Polka Bidco Limited | Wholesale & retail | 2017 |
| Project Polka Topco Limited | Financial services | 2017 |
| PW Fire Limited | Construction | 2024 |
| R. & B.H. Alderson Limited | Construction | 1974 |
| R. Mason Chemicals Limited | Manufacturing | 1960 |
| R.K.P Access Platforms Limited | Administrative & support | 2019 |
| RAW Detail Limited | Real estate | 2004 |
| Return Image Limited | Construction | 2003 |
| Riverside Ices Limited | Wholesale & retail | 2002 |
| Robson Scott Associates Limited | Professional & scientific | 2005 |
| Rogerley Holdings Limited | Financial services | 2021 |
| Ronald Pounder (Turf Accountants) Limited | | 1969 |
| Roundhouse Building Solutions Ltd | Manufacturing | 2004 |
| Russell Mercer Insurance Services Ltd | Financial services | 2002 |
| S&A BM Limited | Wholesale & retail | 2017 |
| S&A Builders Merchants Limited | Wholesale & retail | 2015 |
| S.G. Petch Limited | Wholesale & retail | 1990 |
| S.J. Bowman Draughting Services Ltd. | Professional & scientific | 1999 |
| Seaham Masonic Temple Limited | Real estate | 1986 |
| Secure Electronic Engineering Ltd | Construction | 2009 |
| Sedgefield OUT of School FUN Club Limited | Health & social care | 1995 |
| SG Holdings (North East) Ltd | Administrative & support | 2025 |
| Shark Cleaning Chemicals Ltd | Wholesale & retail | 2022 |
| Sharp & Sons (Jewellers) Limited | Wholesale & retail | 2003 |
| Shop Window ADS (Retail) Limited | Professional & scientific | 2004 |
| Sigsworth Limited | Wholesale & retail | 2025 |
| Simpson Business Solutions Ltd | Real estate | 2002 |
| Singlewood Limited | Wholesale & retail | 2025 |
| Sirius Ecodev (Arbury) Limited | Construction | 2021 |
| Sirius Ecodev (Clapham) Limited | Construction | 2021 |
| Sirius Ecodev (Lambeeth) Limited | Construction | 2021 |
| Sirius Ecodev (PEN YR Heol) Ltd | Construction | 2021 |
| Sirius Ecodev (Toronto) Limited | Construction | 2021 |
| Sirius Ecodev Group Ltd | Professional & scientific | 2017 |
| Sirius Planning Ltd | Professional & scientific | 2016 |
| Sirius Plant Ltd | Administrative & support | 2016 |
| Sirius Renewable Energy Ltd | Energy | 2017 |
| Siriusdev (Miners) Ltd | Construction | 2017 |
| SK Engineering NE Ltd | Professional & scientific | 2018 |
| Sophie Jackson Limited | Wholesale & retail | 2025 |
| Sparkling Hands Ltd | Wholesale & retail | 2015 |
| Spraytech (North East) Limited | Wholesale & retail | 2003 |
| ST Georges Bridge Centre Limited | Arts & recreation | 1997 |
| Stage (Darlington) Community Interest Company | Health & social care | 2007 |
| Stanley Optical Centre Limited | Health & social care | 2004 |
| Station Taxis (Darlington Banktop) Limited | Transport & logistics | 2007 |
| Stephen Russell Hair Design Ltd | Other services | 2002 |
| Stewart Chapman Plumbing & Heating Ltd. | Construction | 2002 |
| Stewart Ward Holdings Limited | Water & waste | 2025 |