| M&H Developments Ltd | Real estate | 2002 |
| M&H Healthcare Ltd | Health & social care | 2015 |
| M&N E-Commerce Limited | Wholesale & retail | 2025 |
| M&N International Limited | Real estate | 2019 |
| M&R Hallam Landscaping, Summer Houses & Sheds Ltd | Manufacturing | 2024 |
| M. P. I. H. Limited | Wholesale & retail | 2003 |
| M. Pattison & Co. Limited | Real estate | 1920 |
| M.A Stanton & Sons Limited | Construction | 2014 |
| M.A. Composites Limited | Manufacturing | 2013 |
| M.A.D.E Marketing Ltd | Wholesale & retail | 2019 |
| M.Beswick Construction Ltd | Construction | 2024 |
| M.Elliot Groundworks NE Ltd | Construction | 2025 |
| M.G.L. Demolition Limited | Construction | 1972 |
| M.G.L. Group Limited | Professional & scientific | 2008 |
| M.Tait Groundworks & Civils Ltd. | Administrative & support | 2005 |
| M5 Automation Limited | Manufacturing | 2007 |
| M5 Design Ltd | Professional & scientific | 2013 |
| M62 Developments Limited | Real estate | 2012 |
| MAB Event Management Limited | Professional & scientific | 2009 |
| MAC Chef Ltd | Hospitality | 2025 |
| Mach European Holdings Ltd | Administrative & support | 2025 |
| Mack Diligence Limited | Professional & scientific | 2011 |
| Mackereth Yard Garden Ltd | Professional & scientific | 2022 |
| MAD About Art | Education | 2001 |
| Madam KD Ltd | Wholesale & retail | 2024 |
| Madden Consultancy Limited | Manufacturing | 2023 |
| Maddison & ORD Property Group Ltd | Real estate | 2026 |
| Maddison Autos Ltd | Wholesale & retail | 2015 |
| Maddison's Property Management Limited | Households | 2025 |
| Made for YOU by Emily Limited | Wholesale & retail | 2025 |
| Mafao Ltd | Wholesale & retail | 2025 |
| Magenta Capital Investments Ltd | Real estate | 2020 |
| Magicworld Trade Ltd | Wholesale & retail | 2025 |
| Magna Civil Engineering Ltd | Construction | 2024 |
| Magna Holdings NE Ltd | Financial services | 2025 |
| Magnahaus Innovation Limited | Construction | 2025 |
| Mahy Trade Ltd | Wholesale & retail | 2025 |
| MAID2CLEAN (Prime) Limited | Other services | 2019 |
| Maintain Construct Property Limited | Construction | 2026 |
| Majara Consulting Ltd | Real estate | 2025 |
| Major & Aspinall Limited | Manufacturing | 1992 |
| Make and Supply Ltd | Transport & logistics | 2020 |
| Makea Ariki Ltd | Manufacturing | 2025 |
| Makepeace Interior Solutions Ltd | Construction | 2024 |
| Mala Art Limited | Wholesale & retail | 2021 |
| Malcom Containers Trading and Rentals Ltd | Wholesale & retail | 2025 |
| Mama Bears World Ltd | Wholesale & retail | 2024 |
| Management Development Centre International Limited | Education | 1993 |
| Mandala Stays Limited | Real estate | 2023 |
| Manhattan Agency and Event Management Limited | Arts & recreation | 2011 |
| Mani Langly Fish Bar Ltd | Hospitality | 2025 |
| MANNY12 Healthcare Ltd | Health & social care | 2024 |
| Manushak Ltd | Manufacturing | 2018 |
| Manuza Limited | Real estate | 2025 |
| Maplegate Holdings Ltd | Wholesale & retail | 2026 |
| Maqasid Ltd | Professional & scientific | 2015 |
| Mara Childrens Nursery and Work Hub Ltd | Administrative & support | 2018 |
| Marbenno LLP | | 2024 |
| Marble Arts TM Limited | Manufacturing | 2001 |
| Marchel Investments Limited | Administrative & support | 2022 |
| Marcin Nager Electrical Engineering Limited | Construction | 2021 |
| Margilou Limited | Real estate | 2016 |
| Marginal Consult Medical Services Ltd | Health & social care | 2009 |
| Margot Films Ltd | Information & communication | 2024 |
| Maria Carney School of Dance Ltd | Arts & recreation | 2019 |
| Marinatex Ltd | Professional & scientific | 2020 |
| Marjac Durham Ltd | Hospitality | 2023 |
| Mark 1 Automotive Refinement Ltd | Wholesale & retail | 2021 |
| Mark Catering Limited | Hospitality | 2024 |
| Mark Helliwell Limited | Construction | 2022 |
| Mark Lyons Plumbing & Heating Ltd | Construction | 2007 |
| Mark Price Footwear Ltd | Wholesale & retail | 2005 |
| Mark Speller Accountancy Ltd | Professional & scientific | 2007 |
| Markalama UK Ltd | Wholesale & retail | 2025 |
| Market Bakery Ltd | Wholesale & retail | 2018 |
| Marl PM Ltd | Professional & scientific | 2024 |
| Marmaris Barbers (Northeast) Ltd | Other services | 2025 |
| Marmaris Barbers - Durham Ltd | Other services | 2020 |
| Marmaris Pro Ltd | Other services | 2026 |
| Marras Festivals Ltd | Arts & recreation | 2017 |
| Marry Cleaning Ltd | Administrative & support | 2025 |
| Marscot Holdings Ltd | Financial services | 2021 |
| Marsden-Knight Limited | Hospitality | 2023 |
| Marshall Marshgate Properties Limited | Real estate | 2024 |
| Marshall T Foods Limited | Hospitality | 2025 |
| Marshall's Golf Limited | Wholesale & retail | 2002 |
| Martha & Lulu Ltd | Hospitality | 2013 |
| Martin Bell Car Sales Limited | Wholesale & retail | 2007 |
| Marwa 27 Food Ltd | Hospitality | 2026 |
| Marzia MS Limited | Wholesale & retail | 2025 |
| Master Haulage Ltd | Transport & logistics | 2024 |
| Master Marketing & Print Limited | Manufacturing | 2020 |
| Mastermind Consultants Limited | Hospitality | 2016 |
| Maternity Expert Witness Ltd | Professional & scientific | 2024 |
| Mathias Group Ltd | Wholesale & retail | 2025 |
| Matrissist Ltd | Wholesale & retail | 2025 |
| Matrix Aero Ltd | Manufacturing | 2024 |
| Matt Duxbury Creative Limited | Professional & scientific | 2024 |
| Matte White Limited | Real estate | 2022 |
| Maughan Reynolds Partnership Ltd | Professional & scientific | 2002 |