| Mason Thomas Limited | Health & social care | 2011 |
| Massraf's Holding Limited | Financial services | 2025 |
| Massraf's Private Practice Ltd | Health & social care | 2007 |
| Massraf's Properties Limited | Real estate | 2015 |
| Matt Hammond HR Ltd | Professional & scientific | 2024 |
| Matthew COX Limited | Manufacturing | 2017 |
| Matthew Wade Decorators Ltd | Construction | 2015 |
| Maverick Multimedia Limited | Information & communication | 2019 |
| Maxey Inns Ltd | Hospitality | 2023 |
| Maxiheat Limited | Construction | 2023 |
| Maxstudios Life Ltd | Construction | 2020 |
| Maze Training Limited | Professional & scientific | 2002 |
| MB Electrical (Stamford) Ltd | Construction | 2013 |
| MB International Consulting Ltd | Professional & scientific | 2025 |
| Mbrewin Sales Ltd | Wholesale & retail | 2023 |
| Mcjc Ltd | Real estate | 2024 |
| MCK Invest Ltd | Real estate | 2023 |
| MD Courier Services (Stamford) Limited | Transport & logistics | 2015 |
| MDA Education Leadership Ltd | Professional & scientific | 2026 |
| MDR Automotive Services Limited | Wholesale & retail | 2025 |
| ME Produce Limited | Wholesale & retail | 2016 |
| Me&Em Graphic Design Ltd | Professional & scientific | 2017 |
| Meadowpark Ridge Ltd | Real estate | 2017 |
| Media City Investments Limited | Wholesale & retail | 2008 |
| Melbourn Bros' | Extraterritorial | 1994 |
| Mello Fitness & Leisure Ltd | Manufacturing | 2020 |
| Menzies Roofing Company Ltd | Construction | 2023 |
| Meridian Group Services Ltd | Health & social care | 2025 |
| Meridian Legal Systems Limited | Information & communication | 1995 |
| Meridian Network Solutions Limited | Information & communication | 2008 |
| Messenger BCR Group Limited | Professional & scientific | 2012 |
| Messenger BCR Limited | Extraterritorial | 2019 |
| Messenger Building Repairs Limited | Construction | 2012 |
| Messenger Conservation Ltd | Extraterritorial | 2012 |
| Messenger Construction Limited | Construction | 2011 |
| Messenger F. D. D. Ltd | Other services | 2017 |
| Messenger Special Projects Ltd | Extraterritorial | 2012 |
| METAB3E Holdings Limited | Financial services | 2025 |
| Method Health Club Ltd | Arts & recreation | 2024 |
| Metregal Limited | Construction | 1978 |
| Mgba Ltd | Professional & scientific | 2005 |
| MI Property Solutions Ltd | Real estate | 2021 |
| Michael Allen Decorators Ltd | Construction | 2016 |
| Michelle Feasey and Co Limited | Professional & scientific | 2002 |
| Midland Land Recovery Limited | Agriculture & fishing | 2025 |
| Might House Construction Ltd | Construction | 2025 |
| Mill Street Marketing Limited | Extraterritorial | 2006 |
| Mill View Motors Ltd | Wholesale & retail | 2014 |
| Millar Dykes Consultants Ltd | Professional & scientific | 2012 |
| Milmont Consulting Limited | Information & communication | 2020 |
| Milstad Ltd | Professional & scientific | 2011 |
| Mimir Advisors Limited | Professional & scientific | 2026 |
| Mind-Body Attunement Ltd | Health & social care | 2026 |
| Mindspace (Stamford) Limited | Health & social care | 2018 |
| Mindthrive Limited | Health & social care | 2024 |
| Minster Group Limited | Extraterritorial | 2019 |
| Mirrorbrand Limited | Professional & scientific | 2005 |
| Miss Moneypenny PA Services Limited | Administrative & support | 2023 |
| Mitchell & Hudson Holdings Ltd | Financial services | 2025 |
| Mitchell & Hudson Limited | Construction | 1979 |
| MK Arts Ltd | Education | 2019 |
| MKA House Trading LLP | | 2022 |
| ML Artifacts Limited | Other services | 2018 |
| MME Property Solutions Ltd | Households | 2023 |
| MN Catering Ltd | Wholesale & retail | 2022 |
| Mnwbarratt Ltd | Wholesale & retail | 2024 |
| MO Advisors Ltd | Information & communication | 2021 |
| Mobile Master Stamford Limited | Wholesale & retail | 2022 |
| Mode Joinery Ltd | Extraterritorial | 2020 |
| Moktan Limited | Hospitality | 2025 |
| Moletrak Ltd | Transport & logistics | 2023 |
| Molinar Consulting Limited | Administrative & support | 2023 |
| Monara Developments Ltd | Other services | 2023 |
| Mono Earth Limited | Energy | 2009 |
| Montague Griffin Ltd | Wholesale & retail | 2024 |
| Montimus Holdings Ltd | Construction | 2024 |
| Moonstone Finance Group Ltd | Financial services | 2018 |
| Moorelake Leisure Limited | Arts & recreation | 2013 |
| Moorelake Limited | Professional & scientific | 2008 |
| Moorelake Projects Limited | Arts & recreation | 2011 |
| Moose Graphics Ltd | Professional & scientific | 2025 |
| Moran Law Consulting Limited | Financial services | 2023 |
| More Limited | Administrative & support | 2000 |
| More Travel Limited | Administrative & support | 2010 |
| Morepeople Limited | Administrative & support | 2000 |
| Moreperformance Limited | Administrative & support | 2008 |
| Moretalent Limited | Administrative & support | 2008 |
| Morethanpolish Limited | Wholesale & retail | 2004 |
| Morgan & French Limited | Wholesale & retail | 2008 |
| MOT Training Group Ltd | Education | 2009 |
| Motofish Ltd | Wholesale & retail | 2025 |
| Motorsavers (Corby) Limited | Wholesale & retail | 1970 |
| MPJ Consulting Limited | Professional & scientific | 2019 |
| Mpsb Ltd | Professional & scientific | 2024 |
| MPU Associates Limited | Professional & scientific | 2023 |
| MR Grafton Limited | Professional & scientific | 2022 |
| MSM Osteopathy Limited | Health & social care | 2010 |
| Mums TV Limited | Information & communication | 2018 |
| Murray Legacy Limited | Real estate | 2026 |
| Mvia UK Ltd | Manufacturing | 2025 |