The comments instanly sparked fear on social media, but a spokesman for No. 10 later said that Mr Johnson had ‘mis-spoke’ and was referring to sewage waste rather than the tap water supply.
During a statement to the House of Commons, the Prime Minister told MPs that it might be possible to be able to track local 'flare ups" of the deadly virus by testing the water supply.
He told MPs: "The intention is that the covid alert system in time will be sufficiently sensitive and flexible so as to detect local flare ups, so that if for example the Covid is detected in the water supply of a certain town or whatever then steps can be taken, or in a school or whatever then, steps can be taken on the spot to deal with that flare up, to keep the R down locally as well as nationally."
A spokesman said Mr Johnson was referring to sewage and said: “That specifically is a reference to sewage or waste water as it’s more politely described.”
There is some evidence showing that genetic material from the new coronavirus can be detected in wastewater and researchers in several countries have begun to assess whether this could be used as an early-warning tool.
There is no evidence that Covid-19 can be found in treated water.