| Allan Oliver Limited | Professional & scientific | 2013 |
| Allani Couriers Ltd | Transport & logistics | 2016 |
| Allen Reid Consultancy Limited | Administrative & support | 2009 |
| Allenby Holdings Ltd | Real estate | 2026 |
| Allerton Bywater Management Company Limited | Households | 2006 |
| Allison Taxis Limited | Transport & logistics | 2014 |
| Allmech Engineering Ltd | Professional & scientific | 2023 |
| Allott & Associates Limited | Professional & scientific | 2002 |
| Allott Target Limited | Financial services | 2023 |
| Almit Group Limited | Professional & scientific | 2008 |
| Almit Metal Finishing Ltd | Manufacturing | 2004 |
| Almit Properties Limited | Real estate | 2008 |
| Alpachino's Pizzeria Ltd | Hospitality | 2018 |
| Alpha (Firth Road) Ltd | Construction | 2023 |
| Alpha Paving Limited | Construction | 2018 |
| Alpinism Limited | Professional & scientific | 2022 |
| ALR Embroidery Limited | Manufacturing | 2024 |
| Alsanobar Trading Ltd | Wholesale & retail | 2025 |
| Alsutanate for Precious Stones and Silver Ltd | Wholesale & retail | 2025 |
| Alternative Scene Ltd | Education | 2026 |
| Alux Electrical Limited | Construction | 2021 |
| Always Smiling Worldwide Community Interest Company | Education | 2018 |
| Alwyn Doors Ltd | Construction | 2023 |
| AM Home Services Ltd | Construction | 2025 |
| Amanda Miller Photography Limited | Professional & scientific | 2022 |
| Amap Consulting LLP | | 2012 |
| Amber Grange (Darlington) Limited | Construction | 2013 |
| Ambient Lite Limited | Information & communication | 2013 |
| Amccormick Electrical Ltd | Construction | 2025 |
| Amcl MC Limited | Financial services | 2018 |
| Amco Edge Ltd | Wholesale & retail | 2025 |
| Amdega Restorations Ltd | Construction | 2023 |
| Amellis Enterprises Ltd | Financial services | 2022 |
| Amellis Properties Ltd | Real estate | 2025 |
| Amiah Limited | Extraterritorial | 2022 |
| Amira Construction Ltd | Construction | 2025 |
| Amirah Local Shop Darlington Ltd | Wholesale & retail | 2025 |
| AMK HGV Ltd | Manufacturing | 2025 |
| AML Service Group Europe Ltd | Administrative & support | 2018 |
| Amlea Ltd | Construction | 2026 |
| Amoorengineering Limited | Manufacturing | 2017 |
| AMP (North East) Holdings Limited | Professional & scientific | 2022 |
| AMP (North East) Limited | Professional & scientific | 1992 |
| AMR Garage Darlington Limited | Wholesale & retail | 2024 |
| Amtech Offshore Ltd | Manufacturing | 2025 |
| AMY SM Limited | Other services | 2020 |
| AMYLEE33 Limited | Information & communication | 2015 |
| Anabas (UK 2) Limited | Administrative & support | 2003 |
| Analytical Testing Laboratories (UK) Limited | Professional & scientific | 2015 |
| Anchorage Mews Estate (III) Management Company Limited | Households | 1994 |
| Andar Financing & Marketing Services Limited | Real estate | 1966 |
| Anderson Technology Ltd | Information & communication | 2024 |
| Andrea Walker Ltd | Transport & logistics | 2017 |
| Andrew Mark Hird Logistics Ltd | Transport & logistics | 2025 |
| Andrew Scaife Limited | Administrative & support | 2010 |
| Andrew Turner Inventions Ltd | Administrative & support | 2013 |
| Andrews Marine Surveys & Inspections Ltd | Administrative & support | 2017 |
| Andy Brown Consultants Ltd | Professional & scientific | 2022 |
| Andy Bruce Windows Limited | Construction | 2024 |
| Andy Coates Enterprises Inspection Limited | Administrative & support | 2017 |
| Andy Hadden Landscaping & Building Services Ltd | Construction | 2024 |
| Andy Harland Motor Services Ltd | Wholesale & retail | 2009 |
| Andy's Candy Bar Ltd | Wholesale & retail | 2019 |
| ANG Trans Limited | Transport & logistics | 2018 |
| Angel Assistance Limited | Professional & scientific | 2000 |
| Angel Superstore Limited | Wholesale & retail | 2018 |
| Angela C Permanent Makeup & Beauty Ltd | Other services | 2022 |
| Angelica Solutions Ltd. | Manufacturing | 2014 |
| Angie's Properties Ltd | Extraterritorial | 2023 |
| Anglin Pegram Limited | Real estate | 2020 |
| Anglotech Group Holdings Limited | Financial services | 2019 |
| Anglotech Group Limited | Administrative & support | 1992 |
| Anglotech Solutions Limited | Administrative & support | 2015 |
| Angus & Co Developments Limited | Construction | 2024 |
| Angy Trans Limited | Transport & logistics | 2014 |
| Anjali Buss Darlo Ltd | Other services | 2019 |
| Annmarie Kirk Tutoring Ltd | Education | 2024 |
| Anstey Joinery and Building Contractors Ltd | Construction | 2013 |
| Antariksh Realtors Limited | Other services | 2025 |
| Anthan Properties Ltd | Real estate | 2001 |
| Anthony LUM & Co Limited | Professional & scientific | 2014 |
| Anthony MAY Ltd | Information & communication | 2023 |
| Anthony's Roofing Solutions Ltd | Construction | 2025 |
| Anti-Corrosion Solutions Limited | Manufacturing | 2025 |
| Antiques Affair Ltd | Wholesale & retail | 2021 |
| Antiques Holdings Ltd | Professional & scientific | 2026 |
| Antoni's Takeaway Ltd | Hospitality | 2026 |
| Antoni's Takeaway NE Ltd | Hospitality | 2019 |
| Anubis GS International Ltd | Administrative & support | 2010 |
| Anwen Investments Limited | Financial services | 2024 |
| Anything BUT Footy Limited | Information & communication | 2020 |
| AO Property Maintenance Solutions Ltd | Construction | 2024 |
| Aoxia Limited | Professional & scientific | 2010 |
| Apcm Legal Group Ltd | Professional & scientific | 2025 |
| Apex Executive Protection Ltd | Administrative & support | 2026 |
| APH Development (NE) Limited | Construction | 2011 |
| Apml Consultancy Ltd | Other services | 2023 |
| Appl Business Support Ltd | Transport & logistics | 2024 |
| Appleshed Consulting Limited | Professional & scientific | 2002 |
| Appleyard Property Services Limited | Professional & scientific | 1974 |