We asked who supplies Thames Water customers' tap water, postcode by postcode. The regulator said no. Thames said yes.

If you live in most of England and Wales, you can type your postcode into a website and see what's in your tap water. If you're one of Thames Water's roughly 10 million customers, until this month, an independent site couldn't do that for you.

The reason was a missing link. Water quality is measured and reported by supply zone, an area of a few thousand to 80,000 people served by the same water. Most companies publish which postcodes fall in which zone. Thames Water didn't, so its quality data couldn't be matched to an address.

So we asked for it, twice.

The regulator's answer

On 18 August 2026 we asked the Drinking Water Inspectorate (DWI), part of Defra, for the Thames Water zone list, a postcode-to-zone lookup, and zone boundaries, under the Environmental Information Regulations 2004.

On 28 August the DWI refused all three, on the grounds that releasing them would harm public safety by revealing information about water supply infrastructure. (Reference EIR2026/18138.)

The letter warned that the information could be combined with other public data to "support hostile reconnaissance, or aid activities intended to disrupt water supplies." Its reasoning was about detailed boundary maps. It did not separately consider the zone names or the postcode list, which were refused along with them.

We asked for an internal review on 4 September. It is due by 30 October 2026, and we'll publish the outcome.

Thames Water's answer

On the same day, 4 September, we sent a narrower request directly to Thames Water. It asked for the zone list and the postcode mapping, and explicitly not for anything about treatment works, pumping stations or pipes.

On 1 October, one day inside the legal deadline, Thames Water disclosed it. That was 215,656 postcodes mapped to 285 supply zones, plus water quality results for 268 zones. Thames did not claim any exception for any part of it.

To be precise about what we got: Thames released the zone list and the postcode mapping. It did not release the zone boundary maps. Those are only available under a licence that would stop us publishing anything built from them, so we didn't sign it. We didn't need it: the postcode mapping does the job.

What the data shows

Thames Water's file holds its latest annual results: about 705,000 individual test results across 268 zones. 123 failed a regulatory standard, about 0.02%. The most common were:

• Coliform bacteria, in 54 zones. Thames's own note says coliforms are environmental bacteria "not generally associated with a risk" to health.
• Nickel, in 20 zones.
• Lead, in 13 zones. For nine of them, Thames's note says elevated lead is "normally associated with supply pipes to, or plumbing within, individual properties, and not with the mains water supply." For the other four, Thames says its investigation traced the failure to lead pipework on the customer's property or its own connecting pipe, or to customer plumbing.

Every Thames postcode now returns its own zone's results at https://www.waterdirectory.co.uk/.

What happens next

The information a regulator called a public safety risk turned out to be something the water company itself was willing to hand over: the zone list and the postcode mapping, in full. We've now sent the same request to every other water company in Great Britain and Northern Ireland. Their replies are due in early November, and we'll publish what each one says, including any refusals.

Chris Pennington, Water Directory. Requests made under the Environmental Information Regulations 2004. Data: Thames Water, EIR-26-27-582.