A group of “citizen scientists” who monitor the waterway said water companies that pollute the Thames “will not be held to account”.

The groups, which include Surfers Against Sewage (SAS) and Water Testing River Thames (WTRT), carried out testing at 24 locations between Oxford and Teddington in west London between mid-March and mid-August.

According to the research group, 47 of the 210 tests carried out showed no health-threatening values ​​for E. coli bacteria or intestinal enterococci.

Thames Water said it was “committed to developing our waterways” and had upgraded 250 sites to prevent overflows.

Water pollution can be caused by illegal discharge of sewage into the river or when the entire system cannot withstand the load, which often happens during heavy rain.

Frances Mateo, who surveyed the river in Teddington on Wednesday, told BBC London she was “shocked” by the condition of the river.

“The other shocking thing is that in my social circle, many people ask me, ‘Is the river safe? Is the river safe for my children to play sports in?'” she said.

“They hesitate because, frankly, we have found that this is not the case.”

The number of hours during which wastewater dumped into the Thames have more than quadrupled in the last yearaccording to figures published by City Hall in January.

Izzy Wittrock said her team has conducted a river quality test every week for the past seven weeks and two of them have returned a “poor rating.”

“This essentially means the water is no longer safe for swimming,” she added.

Ms Mateo said that one of her goals in taking part in the tests was to create “awareness and responsibility”.

In their view, the current system of fining water companies for illegally polluting rivers does not have an effective deterrent effect.

“Until there is real government policy that holds our water companies accountable, I think this is just going to continue,” Ms Mateo said. “So citizens need to come forward, do their own testing and make their voices heard.”

Thames Water plans to take more water from the Thames in Teddington and Replace this water with recycled wastewater The application to renovate the Mogden sewage treatment plant also met with resistance.

Ian McNuff of the campaign group Save Our Land and River said the company “cannot be trusted to do this right”.

A Thames Water spokesman said: “While any runoff from rainfall is unacceptable, the sewerage system has historically been designed to work in this way and prevent sewage from flowing back into people’s homes.”

The 200 site improvements being carried out include a £100 million improvement to the Mogden sewage treatment plant and a £14 million improvement to the Beckton site, which the spokesman said would increase treatment capacity and reduce runoff from rainfall.

They also pointed out that the Thames Tideway Tunnel, London’s new super canal, would help reduce overflows from the Thames – although not all of its tributaries.