London could face water rationing “imminently” because of over-abstraction, over-use and wastage through leaking pipes, a leading environmentalist has warned.

James Wallace, chief executive of River Action, 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% 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.

“When are we going to treat this like it’s an emergency? I appreciate it takes time to dig a reservoir but some of these things, like changing our abstraction requirements, reducing consumer demand, fixing leaky pipes, this is a national crisis and we need to address it that way.”

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.

She said the Government’s recent Plan For Water – which aims to reduce leakages by half by 2050 and lower consumer use from 144 litres per person each day on average to 110 litres – did not go far enough.

In answer to a question about managing water resources, she 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.”

The London Assembly Environment Committee, part of the GLA, asked Ms Ross about the demand for water in the capital.

She said 80% of Thames Water customers already used the Government’s target of 110 litres a day but that 20% used a lot more, mostly for watering gardens.

Thames Water, which is the UK’s largest water company serving 15 million people, wants to build a new 150-billion-litre reservoir in Abingdon, Oxfordshire, which would be the first since the privatisation of water in 1989, but has faced local opposition.

Ms Ross said that had the reservoir been in place last summer, there would have been no need for a hosepipe ban.

She also admitted that many of the company’s pipes were “ageing assets” that should have been replaced, but had not been because customers had not paid for it.

Thames Water, along with other companies, has promised to update its infrastructure to reduce sewage discharges, but it has spent the last few weeks fending off speculation about its financial collapse and nationalisation because of a £14 billion debt pile.

Mr Wallace said this was a result of “rapacious” profiteering from its shareholders, in particular the Australian financial services group Macquarie, which he blamed for loading Thames Water with so much of its debt.

He also called the privatised water system “the Wild West” and accused the Government of allowing “legal and illegal behaviour” to destroy the UK’s water security.

“Let’s not forget that we have a deregulatory political framework that has been in place for decades,” he told the GLA.

“That is the problem. I’m not saying I would do it personally but I can understand why, if you’re in Macquarie between 2006 and 2017, they increased debt from £3.4 billion to £10.8 billion in those few years.

“Why did they do it? They did it to pay out their shareholders. Maybe a little bit of investment, but they did it because they are a rapacious fund that is there to make shareholders rich, they are not there to provide an asset.”

Thames Water failed to meet its target this year on reducing leakages as well as those for sewer flooding, pollution, blockages and customer usage.

Ms Ross said the company was not profitable and this year made after-tax losses of £30 million.

She declined to offer a view on whether privatisation had been a failure, saying she had not “given much thought” to whether the water industry would be in better shape if it had remained publicly owned.