| Milktech Ltd. | Wholesale & retail | 2005 |
| Millen Services UK Limited | Professional & scientific | 2021 |
| Millennium Group PVT Ltd | Wholesale & retail | 2023 |
| Miluneco Limited | Wholesale & retail | 2025 |
| Mindfulshells Ltd | Wholesale & retail | 2026 |
| Minney.Org Limited | Information & communication | 2007 |
| Mint Apartments Property Management Limited | Real estate | 2019 |
| Mint Farm Ltd | Real estate | 2025 |
| Mira Production Ltd | Wholesale & retail | 2025 |
| Miracles Hair and Beauty Lounge Limited | Other services | 2018 |
| Miray Sahin Ltd | Wholesale & retail | 2025 |
| Mirror Finish Detailing Ltd | Administrative & support | 2022 |
| Mirror Mirror (Durham) Limited | Other services | 2014 |
| MIS Leisure Parts Ltd | Wholesale & retail | 2024 |
| Misirce Barbers Ltd | Other services | 2024 |
| Mister D Limited | Real estate | 2023 |
| Misterlin Capital Limited | Real estate | 2026 |
| Misterlin Trading Limited | Wholesale & retail | 2025 |
| Mitchell Management Services Ltd | Administrative & support | 2023 |
| MJ Geosolutions Ltd | Construction | 2025 |
| MJ Jointing Ltd | Construction | 2025 |
| MJ-SERVICES-15 Limited | Administrative & support | 2015 |
| MJB Brickwork Ltd | Construction | 2024 |
| MJC Heating and GAS Services Ltd | Construction | 2018 |
| MJD Lettings Limited | Real estate | 2011 |
| MJF Civil Engineering Ltd | Construction | 2015 |
| MJK Building Ltd | Construction | 2023 |
| MJM Roofing & Cladding Limited | Construction | 2021 |
| MJN Consultancy Services Ltd | Professional & scientific | 2024 |
| MJN Engineering Ltd | Professional & scientific | 2021 |
| MJR Groundworks & Construction Ltd | Construction | 2012 |
| MJS Claims Limited | Transport & logistics | 2006 |
| MJT Solutions Ltd | Professional & scientific | 2022 |
| MK Auto FIX Ltd | Wholesale & retail | 2024 |
| MK Commercial & Domestic Tiling Ltd | Construction | 2023 |
| MKE Asset Management Limited | Financial services | 2022 |
| MKM Strategic Solutions Limited | Information & communication | 2024 |
| Mlcm Holdings Ltd | Financial services | 2020 |
| MM Mart Limited | Wholesale & retail | 2025 |
| Mmae Trade Ltd | Wholesale & retail | 2024 |
| MMJ Technical Services Ltd | Construction | 2025 |
| MMP PMO Consulting Limited | Administrative & support | 2023 |
| Mnfi General Partner Limited | Administrative & support | 2007 |
| Mnfi Investment Holdings Limited | Professional & scientific | 2020 |
| MNM Dining Limited | Hospitality | 2026 |
| MO Barbers (NE) Limited | Other services | 2026 |
| Mobi Ecommerce Ltd. | Wholesale & retail | 2012 |
| Mobie North CIC | Other services | 2025 |
| Mobile School Business Management Ltd | Professional & scientific | 2021 |
| Mobile Tyre Force Ltd | Wholesale & retail | 2023 |
| Mobilewuk Ltd | Manufacturing | 2025 |
| Mobius Training and Consultancy Limited | Professional & scientific | 2019 |
| Mockingbird Durham Ltd | Hospitality | 2026 |
| Modern Energy Solutions Limited | Professional & scientific | 2024 |
| Modex Security Services Ltd. | Administrative & support | 2012 |
| Moloney Investments Limited | Real estate | 2021 |
| Momentum Strategic Planning Ltd | Financial services | 2007 |
| Monckton Coke & Chemical Company Limited(The) | Manufacturing | 1901 |
| Mondaybloom Ltd | Wholesale & retail | 2025 |
| Mondial Investments Ltd | Financial services | 2009 |
| Monic Elvet Limited | Hospitality | 2021 |
| Montiglobal Fractional Solutions Limited | Transport & logistics | 2020 |
| Monty Property Management Ltd | Real estate | 2024 |
| Moordale (North East) Limited | Construction | 2000 |
| More Time (UK) Limited | Administrative & support | 2011 |
| Morgan & Newby Ltd | Professional & scientific | 2016 |
| Morgan Douglas Ltd | Real estate | 2015 |
| Morillo Building Services Limited | Construction | 2017 |
| Morley's Barber's Ltd | Other services | 2025 |
| Morpeth Welding Ltd | Manufacturing | 2019 |
| Morton Sporting Ltd | Arts & recreation | 2018 |
| Most Education Community Interest Company | Administrative & support | 2009 |
| Motion World Ltd | Information & communication | 2025 |
| Motivational Leaders Limited | Administrative & support | 2025 |
| Mottogeek Ltd | Information & communication | 2013 |
| Mount Charles (Mayfair) Limited | Extraterritorial | 2014 |
| Mount Hood (OHA) Ltd | Households | 2021 |
| Mount Oswald Estate Management Limited | Real estate | 2016 |
| Move Fast 365 Limited | Wholesale & retail | 2023 |
| Move the Idea Limited | Administrative & support | 2019 |
| Moven 17 Limited | Real estate | 2020 |
| Movetime Logistics Ltd | Transport & logistics | 2018 |
| Moving and Improving Ltd | Information & communication | 2016 |
| Moving on (Durham) Ltd | Information & communication | 2005 |
| Moymoy Catering Ltd | Hospitality | 2025 |
| MP1 Investments Limited | Financial services | 2022 |
| MPL Management Accountants Ltd | Professional & scientific | 2024 |
| MR B Handyman Services Ltd | Construction | 2025 |
| Mr.Elci Trade Ltd | Wholesale & retail | 2025 |
| Mrblackbrother Ltd | Wholesale & retail | 2025 |
| MRH Renovations Ltd | Construction | 2025 |
| Mrml Holding Ltd | Financial services | 2025 |
| MRP Engineering North East Ltd | Manufacturing | 2014 |
| MRS Interiors Ltd | Wholesale & retail | 2025 |
| Mrsj Holdings Limited | Real estate | 2020 |
| Mrsj Surveys Limited | Construction | 2024 |
| MSD Building Services Ltd. | Construction | 2014 |
| Msged F&S Management Ltd | Public administration | 2024 |
| MT Security Services Ltd. | Professional & scientific | 2021 |
| MTC Construction Limited | Construction | 2007 |