| Marine Conservation Society | Other services | 1990 |
| Marine Consultancy Europe Limited | Professional & scientific | 2025 |
| Mariner Bidco Limited | Financial services | 2025 |
| Mariner Midco 1 Limited | Financial services | 2025 |
| Mariner Midco 2 Limited | Financial services | 2025 |
| Mariner Topco Limited | Financial services | 2025 |
| Mark David Osborne Limited | Wholesale & retail | 2014 |
| Mark Sattin Heating Ltd | Construction | 2022 |
| Marque Approved Limited | Professional & scientific | 2005 |
| Marsh Construction Management Services Ltd | Construction | 2017 |
| Mashuk Miah Limited | Hospitality | 2017 |
| Mason Letting Ltd | Real estate | 2025 |
| Masons Yard (Ross) Ltd | Households | 2019 |
| Mathi Private Limited | Wholesale & retail | 2025 |
| Matoom Limited | Hospitality | 2017 |
| Matrix Training and Coaching Limited | Education | 2009 |
| Maverick Plant Limited | Extraterritorial | 2009 |
| MAX Porter Ltd. | Extraterritorial | 2013 |
| MAY ROE Properties Ltd | Real estate | 2020 |
| MB Industrial Solutions Ltd | Construction | 2025 |
| Mcclymont Consulting Ltd | Professional & scientific | 2013 |
| Mcgoldrick Chiu Limited | Real estate | 2013 |
| MCM (Canal Road) Limited | Households | 2016 |
| MD Payroll & Accounts Ltd | Professional & scientific | 2020 |
| Mdab Business Ltd | Hospitality | 2023 |
| Meadex Rubber Mouldings Limited | Manufacturing | 1999 |
| Medi Bakers Limited | Manufacturing | 2018 |
| Meek Watts Property Lettings Ltd | Real estate | 2021 |
| MER Holdings Ltd | Wholesale & retail | 2020 |
| Meri Mayhew Consulting Ltd | Professional & scientific | 2019 |
| Merrivale Management Limited | Households | 1988 |
| Merrivale Wood Conservation Group CIC | Arts & recreation | 2025 |
| Merton House Holiday Hotel Limited | Health & social care | 1978 |
| Meta Films Limited | Information & communication | 2004 |
| Metalife Limited | Administrative & support | 2010 |
| Metis Partnership Limited | Professional & scientific | 2010 |
| MHS Tipper Hire Limited | Transport & logistics | 2018 |
| MHS Transport & Groundworks Ltd | Construction | 2023 |
| MIA Taylor Training & Aesthetics Ltd | Other services | 2024 |
| Micross Electronics Limited | Manufacturing | 1980 |
| Middle Brampton Farm Limited | Agriculture & fishing | 2025 |
| Milabat Services Limited | Manufacturing | 2023 |
| Mill & Manor Ltd | Wholesale & retail | 2024 |
| Mill Apartments (Ross-On-Wye) Limited | Real estate | 2016 |
| Mind Matters Sport Psychology Ltd | Health & social care | 2020 |
| Minotrap Video Limited | Administrative & support | 2018 |
| MJS Photography Limited | Professional & scientific | 2025 |
| Mkril Limited | Other services | 2023 |
| ML Rees Property Group Ltd | Real estate | 2025 |
| Mobile Infantry Limited | Transport & logistics | 2020 |
| Mona Hotels Ltd | Hospitality | 2016 |
| Monksbury Park Management Company Ltd | Households | 2024 |
| Monksbury Road Management Ltd | Real estate | 2022 |
| Monmouth Memorials Limited | Manufacturing | 2020 |
| Monmouth Xpress Limited | Administrative & support | 2014 |
| Monmouthshire Conservatory Company Limited | Construction | 2004 |
| Monow Limited | Professional & scientific | 2010 |
| Montagu Partners Limited | Financial services | 2022 |
| Montague Brookes Limited | Wholesale & retail | 2025 |
| Moon and Stars Movies Ltd | Information & communication | 2025 |
| Moonstar Properties 2023 Limited | Real estate | 2023 |
| Moraston Grove Management Ltd | Households | 2018 |
| Morgan Boseley Fencing Limited | Construction | 2022 |
| MOT Servicing & Repairs Ltd | Wholesale & retail | 2025 |
| Motley HOG Brewery Limited | Manufacturing | 2018 |
| Motrax Ltd | Information & communication | 2026 |
| Move IT! Move IT Transport Ltd | Transport & logistics | 2014 |
| MSJ 1893 Holdings Limited | Financial services | 2026 |
| MSJ 1893 Limited | Education | 2026 |
| MWL Aesthetics Ltd | Health & social care | 2024 |
| Mwmedicalnegligence Limited | Professional & scientific | 2023 |
| Mxbi.Co.Uk Limited | Information & communication | 2015 |
| My Christmas Ltd | Wholesale & retail | 2025 |
| My Pocket Medic Limited | Information & communication | 2026 |
| My-Architect Studio LLP | | 2020 |
| Myoffers 2019 Limited | Information & communication | 2019 |
| Nahorniak Property Limited | Real estate | 2023 |
| Nails and More at NO.4 Ltd | Other services | 2024 |
| Name That Tune Limited | Extraterritorial | 1998 |
| National Hygiene Services Limited | Water & waste | 2015 |
| Natmain Limited | Households | 1989 |
| Natural Connection Ltd. | Wholesale & retail | 1996 |
| Nature Givers YOU Enjoy CIC | Education | 2025 |
| Natures NO.1 Ltd | Wholesale & retail | 2026 |
| NDT Paraplanning Ltd | Administrative & support | 2025 |
| Neale Lewis Associates Limited | Professional & scientific | 2007 |
| Nerich Developments Limited | Construction | 2020 |
| Nerich Global Mining Industry UK Limited | Wholesale & retail | 2025 |
| Neuroblooms Specialist Services CIC | Education | 2025 |
| New Shelf Limited | Agriculture & fishing | 2012 |
| Newent Auto Centre Ltd | Wholesale & retail | 2019 |
| Newtown Road Regen Limited | Construction | 2021 |
| Nexgen Habitat Limited | Construction | 2018 |
| Nextcore Tech Limited | Information & communication | 2023 |
| Nexus Defence Limited | Wholesale & retail | 2010 |
| NEXUS9 Digital Ltd | Professional & scientific | 2018 |
| Nigel's Windows Ltd | Construction | 2024 |
| Niyah Collaborate Ltd | Professional & scientific | 2018 |
| NJB Holdings Limited | Real estate | 2016 |
| NJB Pittshanger Limited | Other services | 2026 |