What Is the Level of E. coli in UK Water?

Sep 10, 2025

4 min read

Updated: Sep 19, 2025

Last reviewed: September 2026

Quick answer The UK standard for E. coli in drinking water is zero: none should be found in a 100 ml sample, and any detection fails. Illness from UK drinking water is extremely rare. The risk is higher on private supplies (boreholes, wells and springs), in storage tanks, and after heavy rain or work on a supply.

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The UK standard for E. coli

Bacteria	Standard	Source
E. coli	0 per 100 ml	DWI drinking water standards
Enterococci	0 per 100 ml	DWI drinking water standards

Zero per 100 ml means none should be found in a 100 ml sample. Any detection fails the standard.

What E. coli in water means

“E. coli and Enterococci are present in human and animal faeces and are pathogens.” (North Norfolk DC) “Their presence also indicates that other harmful pathogens could be in the water.” (North Norfolk DC)

Coliforms are different: “Coliform bacteria are found widely in the environment.” (North Norfolk DC) They point to “some form of general environmental contamination, such as surface run off into a well, dust in a storage tank, or dirt inside a tap.” (North Norfolk DC)

On illness, NHS inform explains: “Most types of E. coli are harmless and are an important part of a healthy digestive tract. However, some can make you unwell.” (NHS inform) One type, Shiga toxin-producing E. coli (STEC), “can cause severe stomach pain, bloody diarrhoea and kidney failure.” (NHS inform) NHS inform lists “drinking contaminated water from inadequately treated water supplies” (NHS inform) among the ways people catch it.

For mains water, the DWI says: “It is extremely rare for drinking water to cause illness in England and Wales.” (DWI)

Where contamination comes from

Private supplies. “Wells and springs although derived from groundwater, are very often influenced by surface water, and are considered surface water for the purposes of water quality risks.” (DWI)
Heavy rain and flooding. “heavy rainfall can significantly deteriorate the microbiological quality of various water sources” (FSS).
Storage tanks. A loft tank needs a lid that “excludes light and is tightly fitting and securely fastened, so that birds, vermin, and dust cannot get into the water.” (DWI) “Ideally you should only use a tap connected to the mains water supply for drinking, food preparation or teeth cleaning” (DWI).
Taps and treatment. Coliforms can come from “dirt inside a tap” (North Norfolk DC), and a UV lamp “Can stop working and not be noticed if not checked or alarmed.” (North Norfolk DC)

If E. coli is found

Don’t drink it untreated. Boil it or use bottled water. The DWI’s boil water advice: “You should boil it before you drink it, use it to brush your teeth, make ice cubes, prepare food, clean feeding equipment or give it to your pets.” (DWI)
Tell the right people. On mains water, your water company. On a private supply, your council’s environmental health team: “If E. coli and/or Enterococci have been found a Notice is likely to be served by the council.” (North Norfolk DC)
If someone is unwell. “If you believe that your drinking water is causing illness, you should consult a doctor and contact your water company in the first instance.” (DWI) NHS inform says to contact your GP practice urgently if “you or your child has bloody diarrhoea” (NHS inform). “Phone 111 if your GP practice is closed.” (NHS inform)
Confirm and fix. A home screen is a first check, not a lab result. Confirm with your water company, council or an accredited lab, find and fix the source, then retest before drinking untreated. Our full guide: E. coli or coliforms in your water test? What to do next.

Frequently asked questions

What does 0 per 100 ml mean?

None should be found in a 100 ml sample. Any detection fails the standard.

Does boiling kill E. coli?

“Heating water is one of the best ways to kill or inactivate bacteria, viruses or parasites.” (DWI) Then “Remove the water from the heat and allow it to cool naturally.” (DWI)

Is UK tap water safe to drink without boiling?

Normally, yes: “Tap water in the UK is safe to drink without boiling.” (DWI) Private supplies and storage tanks are the exceptions to check.

How often should a private supply be tested?

We suggest a screen at least once a year, in the same month, and after heavy rain, flooding or work on the supply. The Private Supply Planner builds a plan for your supply.