| Approved Scaffolding Limited | Construction | 2025 |
| Apres Food Co. Limited | Hospitality | 2016 |
| Apricity Compliance Ltd | Financial services | 2017 |
| Apssupplies Limited | Wholesale & retail | 2025 |
| Aptum Consulting Ltd | Administrative & support | 2023 |
| Aquashield (North East) Limited | Construction | 2004 |
| Arabesque Construction Limited | Construction | 2024 |
| Arbour & Co Flooring Ltd | Wholesale & retail | 2025 |
| Arc-Tic Milburn Engineers Limited | Manufacturing | 1972 |
| Arc-Tic Milburn Limited | Professional & scientific | 1989 |
| Arches Consulting Ltd | Professional & scientific | 2026 |
| Archon Properties Limited | Real estate | 2024 |
| ARD Solutions Ltd | Professional & scientific | 2021 |
| Ardenter Payroll Ltd | Information & communication | 2018 |
| Area-A Limited | Professional & scientific | 2012 |
| ARG Coatings Ltd | Manufacturing | 2023 |
| Aria House Management Company Limited | Households | 1995 |
| Arifa & Ahyan Limited | Hospitality | 2022 |
| Aris Services Limited | Professional & scientific | 2025 |
| Arise Technologies Limited | Information & communication | 2026 |
| Arish Barber (NE) Ltd | Other services | 2021 |
| Arjw Properties Ltd | Real estate | 2019 |
| Arkle Flooring Ltd | Construction | 2019 |
| ARL (NE) Ltd | Extraterritorial | 2014 |
| Arms of an Angel Properties Ltd | Extraterritorial | 2023 |
| Armstrong Court Residents Company Limited | Households | 1987 |
| Arnco Building Services Ltd | Construction | 2025 |
| Arnett Group Ltd | Wholesale & retail | 2025 |
| Arpent Limited | Construction | 2024 |
| Arronbeth Ltd | Health & social care | 2000 |
| Arronbeth Therapeutic Care Limited | Health & social care | 2021 |
| Arrow Global Solutions Ltd | Other services | 2023 |
| Arrow Heavy Plant Spares Limited | Other services | 2020 |
| Arrow Motor Company Limited | Wholesale & retail | 2019 |
| ARS Global Resources Limited | Wholesale & retail | 2023 |
| Art Infinity Photoprint Ltd | Arts & recreation | 2022 |
| Artemis Financial Services Limited | Financial services | 2023 |
| Arthur Boyd Equine Limited | Wholesale & retail | 2023 |
| Arthur Turner Estates & Leisure Limited | Real estate | 1982 |
| Artistpug Limited | Wholesale & retail | 2025 |
| Arvelli Limited | Hospitality | 2024 |
| Arvelli Restaurant Limited | Hospitality | 2021 |
| AS Agri & Aqua (UK) Limited | Professional & scientific | 2019 |
| AS Mobile Limited | Other services | 2015 |
| ASA Associates North East Ltd | Real estate | 2014 |
| ASA Country Parks Limited | Hospitality | 2012 |
| Ascot Care Agency Limited | Administrative & support | 2014 |
| ASH Vale Services & Logistics Ltd | Administrative & support | 2012 |
| Ashburn Design and Print Ltd | Professional & scientific | 2018 |
| Ashburn Properties Limited | Households | 2020 |
| Ashgill Racing Limited | Agriculture & fishing | 2010 |
| Ashgrove House Developments (Management) Limited | Households | 2004 |
| Ashraya Services Limited | Health & social care | 2023 |
| Ashton Prestige Cars Ltd | Wholesale & retail | 2018 |
| Askew & Co. Consultancy Ltd | Professional & scientific | 2025 |
| ASM PUB Company Ltd | Hospitality | 2019 |
| Asma Star Ltd | Hospitality | 2019 |
| Aspect Signalling Limited | Other services | 2019 |
| Aspire Developments and Building Ltd | Real estate | 2025 |
| Aspire Dynamics Consulting Limited | Information & communication | 2024 |
| Aspire Electrical (Darlington) Limited | Construction | 2020 |
| Aspire Fitness Classes Ltd | Arts & recreation | 2013 |
| Aspire Fitness Studios Ltd | Arts & recreation | 2013 |
| Aspire Home Services Repair Limited | Administrative & support | 2023 |
| Aspire Property Management Services Limited | Construction | 2025 |
| Aspire Psychological Services Ltd | Health & social care | 2015 |
| Asset Management Consultants Ltd | Professional & scientific | 2013 |
| Assured Property Solutions Ltd | Administrative & support | 2026 |
| Astarra Ltd | Manufacturing | 2018 |
| Asthereal Ltd | Health & social care | 2018 |
| Aston Francis Ltd | Other services | 2011 |
| Astral Park Management Company Limited | Households | 2019 |
| ATA Property (Darlington) Ltd | Real estate | 2022 |
| ATD Renovations Ltd | Construction | 2022 |
| ATH Heating Services Ltd | Construction | 2018 |
| Atisha Kadampa Meditation Centre | Other services | 1992 |
| Atkinson Consulting Group Ltd | Professional & scientific | 2025 |
| Atkinson Site Services Ltd | Administrative & support | 2011 |
| Atnib Ltd | Administrative & support | 2026 |
| Atom Talent Solutions Ltd | Administrative & support | 2018 |
| Attica Business Ltd | Other services | 2015 |
| Auckland Consultants Limited | Professional & scientific | 1986 |
| Auckland Retail Ltd | Hospitality | 2024 |
| Audrey Kirby Associates Limited | Health & social care | 2016 |
| Auntee Limited | Professional & scientific | 2025 |
| Aura Haven Limited | Wholesale & retail | 2025 |
| Auratech Innovation Limited | Professional & scientific | 2012 |
| Aurum Knight Restorations Ltd | Administrative & support | 2019 |
| Ausdauer Capital UK Limited | Real estate | 2020 |
| Austec Consulting Limited | Information & communication | 2014 |
| Austec VR Limited | Information & communication | 2024 |
| Austin Sanders Law Firm Limited | Professional & scientific | 2004 |
| Austin Trent Watson Ltd | Administrative & support | 2016 |
| Auto COX Ltd | Transport & logistics | 2025 |
| Auto Electrics (Darlington) Limited | Wholesale & retail | 2025 |
| Auto Rae-Chem Limited | Wholesale & retail | 2011 |
| Auto Skins North East Limited | Wholesale & retail | 2018 |
| Auto Spa Darlington Limited | Administrative & support | 2021 |
| Auto Trackers Ltd | Professional & scientific | 2025 |
| Autodb Ltd | Information & communication | 2020 |