The company warns many of the rivers feeding the Thames have fallen to “shockingly low levels” and calls on customers to cut their usage.
Using alliterative messages from “Care for the Coln” to “Defend the Darent”, the posters target seven communities, asking them to order free water-saving gadgets.
Richard Aylard, sustainability director at Thames, said: “We all need to recognise that the water coming out of our taps comes from our local river or from the aquifer that feeds that river – and the less water we all use the less we need to take from the river.”
The two year period from 2010 to 2011 has been the third driest in the region since records began, he added. “It is no longer a case of if we have a drought this year but rather when, and how bad.”
Flows in the River Lee, which runs from Hertfordshire into northeast London, are 24 per cent of the long-term average, the firm said. The Kennet, in Wiltshire, has dried up completely upstream of Marlborough.
Backing the campaign, Angling Trust chief executive Mark Lloyd said: “Rivers and wetlands are without doubt the most important wildlife areas left in Britain and water levels are vital to support many species that rely on them.”
He added: “Many modern city residents have become completely detached from the fact that the water that comes out of their power shower, or the hose that washes their care, is water that should be keeping a river flowing or a wetland wet.
“People waste vast amounts of water because they don’t pay for it themselves, but there is a huge cost for the community and the environment from their thoughtlessness.”
This call to action comes as environment minister Caroline Spelman hosts a drought summit.
Representatives of environmental groups, major water users and water firms have been invited to crisis talks to tackle the impact of persistent dry conditions.