| Matcha Head Spa Limited | Other services | 2024 |
| Matt Blyth Motorsport Limited | Arts & recreation | 2007 |
| Matthew Ellison Consulting Limited | Other services | 2003 |
| Matthew Hough Limited | Health & social care | 2004 |
| Mawp+Sport Limited | Information & communication | 2004 |
| MAX Wifi Support Ltd | Information & communication | 2022 |
| Maxwifi (UK) Limited | Information & communication | 2002 |
| MAY (Romsey) Ltd | Hospitality | 2025 |
| MC Property Repairs Ltd | Construction | 2019 |
| MC Rudd Property Limited | Construction | 2019 |
| MC Trading South Ltd | Wholesale & retail | 2023 |
| Mcarapps Limited | Information & communication | 2024 |
| Mcclintocks Ltd. | Real estate | 1979 |
| Mccormick Recovery Limited | Wholesale & retail | 2014 |
| Mcculloch Consulting Ltd | Professional & scientific | 2016 |
| MCD Care Limited | Health & social care | 2016 |
| MCD Consultants Ltd | Professional & scientific | 2011 |
| MCD Estates Limited | Real estate | 1998 |
| Mcgill Property Management Limited | Real estate | 2018 |
| Mcnamara Contractors Limited | Construction | 1972 |
| Mcnamara Trading Ltd | Administrative & support | 2026 |
| Mcwilliam Technology Limited | Manufacturing | 2004 |
| MD Romsey Ltd | Wholesale & retail | 2012 |
| Meadebrook Homes Limited | Construction | 2002 |
| Meadow Tree Farm Ltd | Agriculture & fishing | 2022 |
| Meadow View (Management Services) Limited | Households | 2003 |
| Meadowford Consulting Ltd | Financial services | 2023 |
| Meadowland Fencing Limited | Professional & scientific | 2008 |
| Meadowside Financial Consultancy Ltd | Financial services | 2011 |
| Medacx Limited | Health & social care | 2005 |
| Medex Practice Management Limited | Health & social care | 2024 |
| Media 24 Limited | Professional & scientific | 2012 |
| Medical Focus Limited | Information & communication | 1985 |
| Medini Ltd | Professional & scientific | 2022 |
| Medivance Health Limited | Health & social care | 2018 |
| Medivance Healthcare LLP | | 2025 |
| Mega Gifts Ltd | Wholesale & retail | 2025 |
| MEN27 Ltd | Professional & scientific | 2023 |
| Mercer Way (Holdings) Ltd | Real estate | 2023 |
| Mercer Way Stores Limited | Wholesale & retail | 2011 |
| Meridian House RTM Company Ltd | Households | 2023 |
| Merlin Fulfilment Ltd | Transport & logistics | 2016 |
| Merriewood Equestrian Ltd | Arts & recreation | 2021 |
| Metamorph (Training & Development) Limited | Professional & scientific | 1998 |
| Metcalfe Engineering Consultancy Ltd | Professional & scientific | 2023 |
| Metcem Idra Ltd | Manufacturing | 2024 |
| Metsa Ltd | Wholesale & retail | 2026 |
| MFL Investments Limited | Financial services | 2022 |
| MGS Holdco Limited | Financial services | 2023 |
| Mhdocs Limited | Other services | 2018 |
| Mibu Enterprises Ltd | Administrative & support | 2023 |
| Michael the English Tutor Limited | Education | 2016 |
| Michael White Financial Services Limited | Financial services | 2015 |
| Michelmersh Dairy Ltd | Wholesale & retail | 2020 |
| Michelmersh Manor Farm Limited | Agriculture & fishing | 2011 |
| Micro Bar Box Ltd | Wholesale & retail | 2014 |
| Milford Early Years Centre Ltd | Education | 2024 |
| Mill Chase & Haines Limited | Arts & recreation | 2023 |
| Mill Garden Driveway Limited | Real estate | 2023 |
| Miller Clinics Limited | Health & social care | 2009 |
| Minchens Courtyard Development Limited | Construction | 2019 |
| Minimax Close Management Company Limited | Households | 1997 |
| Miriam Fenner Architects Ltd | Professional & scientific | 2019 |
| Misbourne Consultancy Ltd | Professional & scientific | 2025 |
| Misra Limited | Manufacturing | 1985 |
| Mistral Medical Ltd | Health & social care | 2015 |
| Mitchease Limited | Construction | 2026 |
| Mitchell's Transport Limited | Transport & logistics | 2023 |
| Mittell Cars Limited | Wholesale & retail | 2015 |
| MJ Equity Ltd | Real estate | 2025 |
| MJ Equity Trading Ltd | Real estate | 2026 |
| MJF Orthopaedics Limited | Health & social care | 2025 |
| MJR Construction (Southern) Ltd | Construction | 2024 |
| MJS Commercial Ltd | Professional & scientific | 2011 |
| MJS Spinal Limited | Health & social care | 2014 |
| MJS Spinal Services LLP | | 2024 |
| MKV Electrical Limited | Construction | 2025 |
| MMF Sporting Club Ltd | Arts & recreation | 2026 |
| MML RAD Limited | Health & social care | 2021 |
| MMP FD Limited | Construction | 2012 |
| Monkeynut Audiobooks & Sound Ltd | Information & communication | 2020 |
| Mont Caron Ltd | Information & communication | 2015 |
| Moore Secure (South) Limited | Administrative & support | 2002 |
| Moore Surgical Limited | Professional & scientific | 2019 |
| Moresby and Hawkins RTM Company Ltd | Households | 2023 |
| Moreton House Limited | Households | 1984 |
| Morganic Disruptive Technology Limited | Extraterritorial | 2017 |
| Morganic Solutions Limited | Manufacturing | 2008 |
| Morphdex Ltd | Information & communication | 2025 |
| Morton Kloer Consultancy Ltd | Health & social care | 2020 |
| Mothers Laundry Limited | Other services | 2019 |
| Mothline Preservations Ltd | Construction | 2019 |
| Motion Power Services Ltd | Professional & scientific | 2026 |
| Motion Telecom Ltd | Information & communication | 2020 |
| Mottisfont Fly-Fishing Club Limited | Arts & recreation | 2005 |
| Mountbatten School Services Limited | Education | 2011 |
| Move in NOW Ltd | Real estate | 2022 |
| Mozaic Plumbing Company Limited | Construction | 2025 |
| MP Johnson Limited | Professional & scientific | 2023 |
| MR. Michael C Quaye Medico-Legal Ltd | Health & social care | 2024 |