| Marcio IT DON Limited | Wholesale & retail | 2025 |
| Marcio Ltd | Construction | 2025 |
| Marco Marine Hamble Limited | Wholesale & retail | 2017 |
| Marcol Transport Limited | Transport & logistics | 2023 |
| Marcus Greenwood Consulting Limited | Administrative & support | 2014 |
| Marcus J Barnett Limited | Construction | 2003 |
| Marcus James (UK) Ltd | Real estate | 2014 |
| Marcus James Developments Ltd | Real estate | 2018 |
| Marcus James Southampton Ltd | Professional & scientific | 2019 |
| Mardare Transport Ltd | Transport & logistics | 2025 |
| Mardian Ltd | Professional & scientific | 2022 |
| Mardle Products Limited | Wholesale & retail | 1998 |
| Marentin Transport Ltd | Transport & logistics | 2024 |
| Maria 3N Limited | Health & social care | 2016 |
| Maria Dubois Oral Care Ltd | Health & social care | 2024 |
| Maria Martinez Ltd | Professional & scientific | 2009 |
| Maria Samson Ltd | Extraterritorial | 2024 |
| Marian Grace Homes Ltd | Real estate | 2021 |
| Mariandaniel Ltd | Transport & logistics | 2024 |
| Maricel.Care Ltd | Health & social care | 2018 |
| Marico Marine Group Limited | Transport & logistics | 1999 |
| Maricuda Limited | Professional & scientific | 2009 |
| Maricuda Special Projects Ltd | Professional & scientific | 2013 |
| Marie James (UK) Limited | Financial services | 2003 |
| Marie Noel Properties Limited | Real estate | 2023 |
| Marili International Ltd | Wholesale & retail | 2025 |
| Marin Curtain Walling Ltd | Construction | 2018 |
| Marina Developments Limited | Construction | 1972 |
| Marina Projects Limited | Professional & scientific | 2001 |
| Marine & Environmental Services Limited | Professional & scientific | 2013 |
| Marine Accident Digital Investigation Limited | Administrative & support | 2013 |
| Marine and Construction Services Ltd. | Professional & scientific | 2011 |
| Marine and Property Services Ltd | Manufacturing | 2019 |
| Marine and Risk Consultants Limited | Transport & logistics | 1999 |
| Marine and Wild Ltd | Wholesale & retail | 2021 |
| Marine Carpentry Limited | Construction | 2021 |
| Marine Clean Limited | Administrative & support | 1988 |
| Marine Communication and Electronics Limited | Manufacturing | 1987 |
| Marine Computing International Limited | Professional & scientific | 1999 |
| Marine Controls & Autonomy Limited | Professional & scientific | 2019 |
| Marine Design Hub Ltd | Professional & scientific | 2021 |
| Marine Drive Engineering Ltd | Manufacturing | 2008 |
| Marine Dynamics Ltd | Manufacturing | 2025 |
| Marine Electrical International Limited | Construction | 2024 |
| Marine Electronic Systems Limited | Professional & scientific | 1977 |
| Marine Energy Consulting Limited | Professional & scientific | 2021 |
| Marine Events Limited | Arts & recreation | 2003 |
| Marine Factors Limited | Wholesale & retail | 2010 |
| Marine Firewatch Services Limited | Extraterritorial | 2015 |
| Marine Glass Maintenance International Limited | Professional & scientific | 1991 |
| Marine Labour Services MLS Ltd | Manufacturing | 2014 |
| Marine Machine U.K. Limited | Manufacturing | 2017 |
| Marine Power & Servicing Limited | Financial services | 2017 |
| Marine Power Vessels Limited | Professional & scientific | 2024 |
| Marine Resources Recruitment Ltd | Administrative & support | 2003 |
| Marine Results International Limited | Manufacturing | 2009 |
| Marine Services Holdings Limited | Financial services | 2026 |
| Marine Shrink Wrapping Ltd | Administrative & support | 2026 |
| Marine Star Ltd | Administrative & support | 2024 |
| Marine Tech Systems Limited | Professional & scientific | 2016 |
| Marine Valeting Services Limited | Administrative & support | 2003 |
| Marine Vehicle Heating Limited | Wholesale & retail | 2012 |
| Marine Wood Solutions Limited | Construction | 2016 |
| MARINE3D Ltd | Manufacturing | 2019 |
| Marineguard Systems Limited | Administrative & support | 1998 |
| Marinejunction Limited | Professional & scientific | 2023 |
| Mariner Group Holdings Ltd | Real estate | 2024 |
| Mariner Interiors Ltd | Construction | 2015 |
| Mariner Plumbing Ltd | Construction | 2017 |
| Mariner Projects Limited | Construction | 2021 |
| Mariners Court Residents' Association Limited | Households | 1977 |
| Mariners Vault Ltd | Wholesale & retail | 2025 |
| Mario Cars Centre Ltd | Wholesale & retail | 2018 |
| Mario Lopes Solutions Ltd | Hospitality | 2019 |
| Mario M&F Limited | Construction | 2018 |
| Mario&Amelia Ltd | Transport & logistics | 2026 |
| Mariofit Ltd | Construction | 2009 |
| Mariposa Kitchen & Bathroom Southern Limited | Construction | 2025 |
| Mariposa Technologies Limited | Extraterritorial | 2014 |
| Maris Architecture Ltd | Professional & scientific | 2024 |
| Mariscan Ltd | Professional & scientific | 2026 |
| Marisu Limited | Information & communication | 2026 |
| Maritech Construction Ltd | Professional & scientific | 2015 |
| Mariti Limited | Health & social care | 2014 |
| Maritime Archaeology Limited | Professional & scientific | 2003 |
| Maritime Archaeology Trust | Professional & scientific | 1989 |
| Maritime Chambers RTM Company Limited | Households | 2013 |
| Maritime Filming UK Ltd | Transport & logistics | 2018 |
| Maritime Insight Solutions Ltd | Professional & scientific | 2026 |
| Maritime Media Ltd | Information & communication | 2026 |
| Maritime Ocean Charters Ltd | Other services | 2023 |
| Maritime Park (Newtown) Limited | Real estate | 2026 |
| Maritime Solent Limited | Other services | 2021 |
| Maritime Systems Ltd | Professional & scientific | 2019 |
| Maritime Yacht Services Ltd | Manufacturing | 2016 |
| Marius Building Group Ltd | Construction | 2023 |
| Marius Purece Ltd | Transport & logistics | 2021 |
| Marius Stan Ltd | Transport & logistics | 2019 |
| Marius&Mihai Logistic Ltd | Wholesale & retail | 2022 |
| Mark and Neil Limited | Real estate | 2002 |