| Malaikat Enterprises Limited | Professional & scientific | 2012 |
| Malcolm Williams Consulting Limited | Financial services | 2012 |
| Malgari Ltd | Hospitality | 2025 |
| Mandy B'S Ltd | Agriculture & fishing | 2025 |
| Mannor Homes Limited | Construction | 2017 |
| Mansant Software Limited | Information & communication | 2023 |
| Manufacturing Intelligence Ltd | Manufacturing | 2018 |
| Many Tears Animal Rescue | Other services | 2007 |
| MAR Vista Properties Ltd | Real estate | 2026 |
| Marc Hague Consultancy Ltd | Manufacturing | 2025 |
| Marc Howells Tutoring Services Ltd | Education | 2026 |
| Mardie Holdings Limited | Construction | 2016 |
| Marelli Automotive Systems UK Limited | Manufacturing | 1987 |
| Marie-Lou Wellbeing Ltd | Other services | 2026 |
| Mario's Motors Limited | Wholesale & retail | 2021 |
| Mark Howard Limited | Construction | 2003 |
| Mark Lloyd Mortgages Ltd | Financial services | 2022 |
| Mark Owen Limited | Construction | 1998 |
| Marketing Moguls Limited | Professional & scientific | 2024 |
| Marmaro Ltd | Hospitality | 2026 |
| Martin Kemp Antiques Limited | Wholesale & retail | 2016 |
| Martin the Plumber Limited | Extraterritorial | 2015 |
| Martyn Palfreman Consulting Ltd | Professional & scientific | 2021 |
| Martyn Phillips Properties Limited | Real estate | 2021 |
| Marzano's Mobile Bar Ltd | Manufacturing | 2021 |
| Mason Hates Kindergarten Limited | Education | 2022 |
| Master Minibus Hire Limited | Transport & logistics | 2022 |
| Matchroom Properties Ltd | Real estate | 2023 |
| Mavers Building Contractor Limited | Construction | 2020 |
| Mavers Building Services Limited | Construction | 2017 |
| Maxx Llanelli Limited | Wholesale & retail | 2014 |
| MAZ Electronics Ltd | Other services | 2025 |
| MB Project Management Ltd | Extraterritorial | 2007 |
| MCK Cars Limited | Transport & logistics | 2016 |
| MCK Vans Limited | Wholesale & retail | 2017 |
| Mckenna Carpentry Ltd | Construction | 2025 |
| Mckenna LOW Loader & Haulage Ltd | Extraterritorial | 2025 |
| Mckenna Waste Ltd | Water & waste | 2019 |
| Mckimm Consulting & Associates Ltd | Administrative & support | 2016 |
| Mcnamara Offshore Services Ltd | Construction | 2026 |
| MD Drone Services Ltd | Information & communication | 2018 |
| MDB Asset Holdings Limited | Financial services | 2025 |
| MDT Transport Ltd | Transport & logistics | 2023 |
| Mech TV Limited | Information & communication | 2021 |
| MEDI25 Properties Limited | Real estate | 2023 |
| Megland Properties Limited | Real estate | 2002 |
| Megs ZEN Escape Ltd | Other services | 2025 |
| Melbourne House Care Home Limited | Health & social care | 2002 |
| Melody Shads Entertainment Ltd | Education | 2024 |
| Menter CWM Gwendraeth Elli Cyfyngedig | Administrative & support | 1993 |
| Menter MIN NOS Cyfyngedig | Health & social care | 2008 |
| Menter Plant Y CWM Cyfyngedig | Education | 2008 |
| Menter Taliesin Ltd. | Education | 2022 |
| Mentorn Broadcasting Limited | Information & communication | 1997 |
| Mentorn Films Limited | Information & communication | 1992 |
| Mentorn Group Limited | Information & communication | 1994 |
| Mentorn International Limited | Information & communication | 1992 |
| Mentorn Limited | Information & communication | 2002 |
| Mentorn Media Limited | Information & communication | 1988 |
| Mentorn Ufo's Limited | Information & communication | 2005 |
| Mentrus Energy Limited | Professional & scientific | 2013 |
| Mentrus Truss & Joist Ltd | Construction | 2019 |
| Merge Services Limited | Construction | 2007 |
| Merge Timberframe Limited | Professional & scientific | 2013 |
| Merger Media Ltd | Information & communication | 2016 |
| Merlin's Coach Tours Ltd | Transport & logistics | 2007 |
| MES Bach Ltd | Health & social care | 2025 |
| Metro Electrical (Wales) Ltd | Construction | 2014 |
| MG1 Developments Ltd | Real estate | 2018 |
| Midas Leasing Limited | Financial services | 1996 |
| Migel's Limited | Hospitality | 2017 |
| Mighty Media Productions Ltd | Extraterritorial | 2023 |
| Mike Clarke Printing Limited | Manufacturing | 1988 |
| Mike Rickwood & Sons Limited | Wholesale & retail | 2008 |
| Mike West Plumbing and GAS Services Ltd | Construction | 2020 |
| Milex Properties Ltd | Real estate | 2022 |
| Mill & Arch Properties Ltd | Real estate | 2025 |
| Millard Advisory Partners Ltd | Construction | 2026 |
| Millard Construction Limited | Manufacturing | 2014 |
| Millard Solutions Ltd | Construction | 2015 |
| Mimi Developments (2018) Ltd | Construction | 2018 |
| Mind Motion and ME Limited | Health & social care | 2025 |
| Mindaugas Brezgys Transport Limited | Administrative & support | 2015 |
| Misa Ltd | Hospitality | 2025 |
| Mizrak Catering Ltd | Hospitality | 2019 |
| MJJ Assessment Ltd | Education | 2019 |
| Mjsc Property Ltd | Real estate | 2021 |
| MKO Access Ltd | Professional & scientific | 2025 |
| Mm&Dm Building Services Limited | Construction | 2026 |
| MMJ Lettings Limited | Real estate | 2007 |
| MMR Holdings Limited | Real estate | 1997 |
| MNP Homes Limited | Real estate | 2023 |
| Moalla Consultancy Ltd | Health & social care | 2014 |
| Mobile Cctv Services Limited | Administrative & support | 2025 |
| Mobile Sport Limited | Information & communication | 1984 |
| Modular Integrated Systems Ltd | Construction | 2019 |
| Mohamed Transport Ltd | Transport & logistics | 2026 |
| Monkey Tree Care Ltd | Professional & scientific | 2015 |
| Montgomery Consultancy Ltd | Agriculture & fishing | 2020 |
| Mordi Property Management Company Ltd | Real estate | 2023 |