| M Roberts Property Investment Ltd | Real estate | 2017 |
| M Roberts Property Service Ltd | Real estate | 2017 |
| M Stevens Plant Ltd | Administrative & support | 2025 |
| M Stevens Welcome Ltd | Hospitality | 2018 |
| M Tyler Ltd | Construction | 2024 |
| M W Williams & SON Contracting Limited | Construction | 2026 |
| M W Williams & SON Landscapes Limited | Administrative & support | 2024 |
| M Y ALI Limited | Hospitality | 2025 |
| M&A International Holdings Ltd | Construction | 2021 |
| M&C Productions Ltd | Administrative & support | 2023 |
| M&D Build & Lettings Ltd | Real estate | 2025 |
| M&E Properties Ltd | Real estate | 2023 |
| M&J Exterior Cleaning Limited | Administrative & support | 2023 |
| M&J Travel Ltd | Transport & logistics | 2009 |
| M&K Property Group Limited | Real estate | 2025 |
| M&M Build Ltd | Construction | 2025 |
| M&M Electronics Ltd | Wholesale & retail | 2026 |
| M&M Property and Developments Ltd | Real estate | 2025 |
| M&S Leisure & Hospitality Group Limited | Hospitality | 2025 |
| M&S Services 4 YOU Limited | Professional & scientific | 2018 |
| M&T Aban Limited | Health & social care | 2024 |
| M&V Auto Repairs Ltd | Wholesale & retail | 2017 |
| M-Dau Architectural Design & Property Services Management Ltd | Professional & scientific | 2025 |
| M. and a. Romanello Limited | Wholesale & retail | 1977 |
| M. C. & SON Properties Limited | Construction | 2015 |
| M. G. Osteopathy Limited | Health & social care | 2003 |
| M. Monni Fish Bar Ltd | Hospitality | 2025 |
| M.A. Hartley Roofing Contractors Ltd | Construction | 2001 |
| M.A.C.K Building Ltd | Construction | 2024 |
| M.A.G.Engineering Company Limited | Administrative & support | 1970 |
| M.A.R. Building Contractors (Swansea) Limited | | 1985 |
| M.B.S Quantity Surveying Consultancy & Project Management Services Ltd | Professional & scientific | 2017 |
| M.B.Telecommunications Limited | Information & communication | 2023 |
| M.C.L. Bricklaying Ltd | Construction | 2003 |
| M.E.P. Building Services Consultants Ltd | Professional & scientific | 2020 |
| M.J. T.V. Repairs Ltd | Manufacturing | 2003 |
| M.M.P Marine & Inspection Limited | Other services | 2019 |
| M.R. Fussell Limited | Wholesale & retail | 1996 |
| M16 Extreme Ltd | Wholesale & retail | 2017 |
| M31 Consultancy Ltd | Information & communication | 2022 |
| M4 Financial Group Limited | Financial services | 2012 |
| MA2 Developments Ltd | Construction | 2018 |
| MAB Properties Ltd | Real estate | 2019 |
| Mabbolt Limited | Hospitality | 2013 |
| MAC Electrical Services Limited | Construction | 2021 |
| MAC Signs UK Ltd | Manufacturing | 2007 |
| Mac-N-Cheezy Ltd | Hospitality | 2025 |
| Machine Tool Services Limited | Manufacturing | 1994 |
| Maclean Health Ltd | Health & social care | 2019 |
| Macmillan Digital Services Ltd | Information & communication | 2025 |
| Macora Ltd | Wholesale & retail | 2025 |
| Macpherson Process Solutions Limited | Information & communication | 2003 |
| Macro Shack Ltd | Hospitality | 2024 |
| Macsen Limited | Professional & scientific | 1998 |
| Macswis Ryal Limited | Hospitality | 2014 |
| MAD 4 Fitness Limited | Arts & recreation | 2008 |
| MAD Concepts Ltd | Arts & recreation | 2005 |
| MAD Dog Clothing Limited | Wholesale & retail | 2009 |
| MAD LAB Fitness Ltd | Arts & recreation | 2012 |
| MAD Macsen Ltd. | Arts & recreation | 2015 |
| MAD007 Limited | Professional & scientific | 2025 |
| Madam Patti Limited | Hospitality | 2020 |
| Madame Foner Limited | Wholesale & retail | 1955 |
| Madbuns Gorseinon Ltd | Hospitality | 2024 |
| Madbuns Ltd | Hospitality | 2021 |
| Madden & Winkett Developments Ltd | Construction | 2007 |
| Maddocks'Cakes From Wales Limited | Manufacturing | 2006 |
| Made Happy Limited | Construction | 2020 |
| Madina Butcher & Grocery Ltd | Wholesale & retail | 2026 |
| Madineni Stores Ltd | Wholesale & retail | 2024 |
| Madog Outdoors Ltd | Professional & scientific | 2020 |
| MADOG30 Limited | Mining & quarrying | 2020 |
| Madrick Southwest Ltd | Wholesale & retail | 2021 |
| MAE Grwp Hale CYF | Financial services | 2024 |
| MAE Medical and Legal Limited | Health & social care | 2021 |
| Maendy Holdings Ltd. | Real estate | 2023 |
| Maerdy Plant Limited | Construction | 2005 |
| Maes Psychology Ltd | Health & social care | 2025 |
| Maes-Y-Bryn Ltd | Real estate | 2023 |
| Maesteg House Care Home Limited | Health & social care | 2018 |
| MAG Specialist Training Consultancy Ltd | Professional & scientific | 2021 |
| Magazine Trader Ltd | Wholesale & retail | 2025 |
| Magdent Limited | Health & social care | 2023 |
| Magiboards Limited | Manufacturing | 2005 |
| Magic Air Inflatables Ltd | Manufacturing | 2018 |
| Magic Bean Developments Limited | Construction | 2024 |
| Magic Brands Corporation Limited | Professional & scientific | 2015 |
| Magic Brands Holdings Limited | Financial services | 2024 |
| Magic Brands Limited | Professional & scientific | 2020 |
| Magic Consultants Limited | Real estate | 2020 |
| Magic Racing Co Limited | Arts & recreation | 2018 |
| Magic Restaurant Group Limited | Hospitality | 2020 |
| Magleam Clean Limited | Administrative & support | 2025 |
| Magma Moulding Ltd | Manufacturing | 2012 |
| Magnetrade Limited | Real estate | 1997 |
| Magpie BD Limited | Hospitality | 2024 |
| Magraa Business Hub Limited | Real estate | 2026 |
| Magraa Properties Limited | Real estate | 2019 |
| Magraa Property Group Limited | Financial services | 2021 |
| Mahmood Pharma Ltd | Wholesale & retail | 2021 |