Water companies have performance commitments on issues such as leakage.

It is illegal to release raw sewage into rivers and seas during “normal” weather conditions, but firms are allowed to do so when it rains heavily, to prevent homes being flooded.

Thames is the UK’s largest water company, supplying water and wastewater services to 16 million customers across London and parts of southern England.

Speaking to the BBC’s Big Boss Interview podcast, Weston said the company treats 4.3 billion litres of waste a day, and “99.5% of the time” it gets treated successfully, although “sometimes something goes wrong”.

“We want to do better when it comes to pollutions,” he said, but added, “the targets that the company is expected to make are not realistic.

“[For example], we have to hit a certain level of leakage, but it is so far in excess of what we are capable of doing, I think anyone would be capable of doing, however much money you invested, that it is not going to be achievable.”

The chance of getting to zero pollution was “very, very slim”, he added.

Ofwat told the BBC: “With around a fifth of water put into supply still lost through leakage, companies must deliver on the commitments they have been funded to achieve.

“Water company targets are intended to be ambitious and drive better outcomes for customers and the environment.”

A spokesperson for the Environment Agency said: “[We] and the public expect Thames Water to comply with the law,” adding that the agency would continue to hold companies to account where performance falls short.

James Wallace, chief executive of campaign group River Action, said: “Thames Water’s tactics of opacity and deflection fool no-one. Telling the public to save water while this wasteful profit-obsessed corporation leaks 570 million litres of treated drinking water every day is offensive.

“There is nothing ‘realistic’ about accepting sewage pollution as inevitable. The choice is simple: keep propping up a failed privatised financial model, or put Thames Water into special administration and rebuild it as a public utility serving customers, rivers and the public.”