Water could be rationed “imminently” across north London, a leading environmentalist has warned.

James Wallace, chief executive of River Action, said the ration could be enforced because of over-abstraction, over-use and wastage through leaking pipes.

The environmentalist said the chalk streams that provided the capital with drinking water were at risk of drying up and that the situation needed to be treated as an emergency.

Speaking to members of the Greater London Authority (GLA), Mr Wallace said: “When we look at water security, look at the Chilterns. The Chilterns feed north London.

“Now many streams are permanently dry and some are only 25 per cent of normal flow rates. This means that we are going to be seeing rationing of water in north London imminently. We’re not talking 20, 30, 40 years, we’re talking imminently.”

He added that “this is a national crisis” and needs to be addressed.

In its annual report published this week, Thames Water said it lost 602.2 million litres a day for 2022/23 based on a three-year rolling average, although the company’s interim chief executive Cathryn Ross clarified to the GLA that one-third of this was actually unmeasured consumer use.

Just weeks ago, Thames Water assured that a “hosepipe ban” for this summer was unlikely, as the wet weather in the spring put the supplier “in a good place”.

In answer to a question about managing water resources, Ms Ross said: “Yes, there is more we need to do.

“But we actually need to change our national conversation about water, we need to change how we think about water.

“We need to understand, for example, that London has more or less the same rainfall as Jerusalem and we’re not living in a wet country where we can take water for granted.”