Health concerns have been raised over the presence of potentially toxic algae in the River Thames' first official bathing spot, just three months after the site won the title.

Swimmers have been warned to stay out of the new designated bathing water site in the Thames at Ham and Kingston, after the Environment Agency (EA) found a bloom of potentially toxic blue-green algae around the area. High concentrations of blue-green algae can produce toxins harmful to humans, pets and wildlife.

Richmond Council has advised visitors not to swim in or around the spot until further notice. It said pets should be kept out of the water and people should avoid contact with visible algae near the riverbank.

The stretch of the Thames between the Half Mile Tree and the YMCA Hawker Centre was officially assigned as London's first bathing water site on May 15. It has not yet been classified for water quality as samples are still being taken, with classification due to be issued after bathing season ends on September 30.

The EA will be taking water samples over the next 24 hours to confirm the extent of the algal bloom, which is at greater risk of growing in hot, dry weather.

Campaigners from Save Our Lands and River (SOLAR) expressed concern for public health over the findings, particularly after an EA inspection on May 11 found the nearby Hogsmill Sewage Treatment Works in breach of its environmental permit on four counts – including inadequate storm sewage storage.

Ian McNuff, from SOLAR, told the Local Democracy Reporting Service (LDRS): "It's deeply concerning, but sadly not surprising, that London's new bathing site at Ham and Kingston has been hit by a toxic algae warning just weeks after opening.

"Toxic algae experiences turbo-charged growth with sewage overflows in warm, sunny conditions – exactly what you'd expect just downstream from the Hogsmill that has failed its recent environment audit."

The bathing spot is also near Thames Water's target location for replacing up to 75million litres of water a day with treated wastewater during drought, known as the Teddington direct river abstraction project.

Thames Water's plan to ensure London has enough drinking water during times of prolonged dry weather would see huge volumes taken from the Thames above Teddington Weir and transferred along a new pipeline to an existing underground tunnel.

This water would join the Lee Valley reservoirs, ready to become drinking water. It would be replaced with "highly-treated" wastewater from Mogden Sewage Treatment Works via a new tunnel.

SOLAR has been campaigning against the scheme since it was announced in 2023, over concerns for public and environmental health.

Ian raised concerns for the bathing spot if the scheme proceeds as it would be very close to where the treated wastewater would be pumped into the river, although Thames Water said it would flow downstream away from the site.

He said: "Families were promised a safe place to swim and enjoy the river, not a public health gamble."

A Thames Water spokesperson said: "We would like to reassure river users that our proposed Teddington direct river abstraction project would not deteriorate water quality and would be required to meet strict environmental and water quality standards set by the Environment Agency.

"The new treated water outfall would also be located downstream of the proposed bathing water site, meaning recycled water would flow downstream over Teddington Weir into the tidal Thames."

SOLAR is running a petition demanding all new wastewater treatment infrastructure meets stricter standards as demanded in EU countries, effectively adding a fourth stage of treatment to remove micropollutants.

The petition has attracted more than 42,850 signatures and needs to hit 100,000 by November 1 to be considered for a debate in Parliament. It can be found here: https://petition.parliament.uk/petitions/763721.

An Environment Agency spokesperson said: "We have identified blue-green algae on the lower River Thames including the Ham and Kingston designated bathing water.

"We are working with the Royal Borough of Kingston Upon Thames, who have put signage in this location to warn people that it is not safe to swim. An advice against bathing notice has been activated on the SWIMINFO website.

"It is likely that the section of the River Thames between Hampton and Kingston has also been impacted. It is important that members of public do not enter the Thames along this stretch. It is also vitally important to keep dogs out of the water."

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