Thames Water supplies most of London (76.8% of its postcodes), and the quality of that water is regulated by the Drinking Water Inspectorate in England and Wales, per ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries (read 26 September 2026).
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
Drinking Water Inspectorate (DWI) enforces the UK drinking water quality regulations in England and Wales. Figures it classifies or regulates on this page cover England and Wales only.
The official answer
London is supplied by Thames Water, whose water is regulated by Drinking Water Inspectorate under the UK drinking water quality regulations.
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
Who supplies London
Thames Water's supply area (76.8% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 139053 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
Affinity Water's supply area (13.4% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 24419 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
Northumbrian Water's supply area (5.4% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 9955 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
SES Water's supply area (3.2% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 5885 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
On or near the boundary between Affinity Water and Thames Water (0.2% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 486 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
On or near the boundary between SES Water and Thames Water (0.1% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 227 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
On or near the boundary between Northumbrian Water and Thames Water (under 0.1% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 172 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
Not placed in any regional water company's supply area (0.4% of the 181,040 live postcodes in Greater London (its 33 local authorities)): 843 postcodes
Source: ONS Postcode Directory (May 2026), each postcode placed in the water companies' supply-area boundaries — read 26 September 2026Contains OS data © Crown copyright and database right 2026Contains Royal Mail data © Royal Mail copyright and database right 2026Source: Office for National Statistics licensed under the Open Government Licence v.3.0Water company boundaries © Water UK, licensed under CC BY 4.0
What an independent test shows
We have not tested London’s water ourselves. Nothing on this page is our own measurement. You can have your own supply tested.