Toxic Algae Discovered at London’s First Official Thames Bathing Site

Potentially toxic blue-green algae has been detected at London’s first official bathing water site near Ham and Kingston. Find out what visitors need to know.

London’s first official bathing water site has been hit by a warning after potentially toxic blue-green algae was discovered in the River Thames near Ham and Kingston. The Environment Agency has advised that the algae could pose risks to people, pets and wildlife, prompting local authorities to tell visitors not to enter the water until further notice.

The discovery comes only three months after the west London stretch of the Thames received official bathing water status. The development has raised fresh questions about river safety, water quality and the challenges facing efforts to make parts of the Thames more accessible for recreational swimming.

Toxic blue-green algae found in the River Thames

The Environment Agency detected a bloom of potentially toxic blue-green algae in the Ham and Kingston bathing area. Also known as cyanobacteria, blue-green algae can multiply rapidly when conditions are warm, sunny and dry.

Although not every bloom produces harmful toxins, high concentrations can create a health risk. Exposure can affect humans and animals, particularly if contaminated water is swallowed or if people come into close contact with the algae.

The Environment Agency said further water samples would be collected to establish the extent of the bloom and assess the situation more closely.

Richmond Council has advised members of the public not to swim in or around the affected area until further notice. People have also been urged to keep dogs and other pets away from the water and avoid touching visible algae along the riverbank.

The warning is particularly important during periods of hot weather, when conditions can encourage blue-green algae to grow more quickly.

Where is the affected bathing area?

The official bathing water site covers a section of the River Thames between the Half Mile Tree and the YMCA Hawker Centre. It was formally designated as London’s first official bathing water site in May 2026.

The site is located around the Ham and Kingston area of southwest London, making it an important location for people hoping to swim, paddle or spend time beside the river.

However, the area has not yet received an official water quality classification. Authorities are continuing to collect samples during the bathing season, which runs until 30 September. A classification is expected after the end of the season.

The Environment Agency has also indicated that the affected area may extend beyond the immediate bathing site. The section of the Thames between Hampton and Kingston could also have been affected by the algae bloom.

For visitors, this means caution is advisable even outside the formally designated bathing zone.

What should swimmers and dog owners do?

People planning to visit the Thames should follow the latest advice from local authorities and the Environment Agency before entering the water.

If visible blue-green algae is present, people should avoid swimming or coming into contact with it. Dogs should also be kept away because animals can become seriously ill after exposure to toxic algae.

Symptoms following exposure can vary depending on the type and concentration of toxins involved. Anyone who becomes unwell after contact with potentially contaminated water should seek appropriate medical advice.

The Environment Agency has activated an advice against bathing notice on its SWIMINFO service. Warning signs have also been installed locally to alert visitors that swimming is currently considered unsafe.

The situation highlights why checking water quality information before recreational swimming is particularly important, even at officially designated bathing water locations.

Why does hot weather increase the risk?

Blue-green algae can thrive when environmental conditions are favourable. Warm temperatures, bright sunlight and periods of dry weather can contribute to rapid growth.

The risk is not limited to lakes and ponds. Rivers and other waterways can also experience algal blooms when conditions allow cyanobacteria to multiply.

Environmental campaigners have raised concerns about the potential relationship between warm weather, sewage pollution and algae growth in the River Thames.

The campaign group Save Our Lands and Rivers, known as Solar, has argued that sewage overflows and pollution could contribute to conditions that encourage algae. The group has also raised concerns about water quality near the Hogsmill River.

These claims are part of a wider debate about the health of London's waterways and the pressures placed on the River Thames by pollution, drought and increasing demand for water.

Thames Water project adds to water quality debate

The discovery of toxic algae comes as another major water project is being considered in the same wider area.

Thames Water is developing plans known as the Teddington Direct River Abstraction project. The scheme is intended to help maintain water supplies during droughts by taking water from the River Thames and transferring it to a reservoir.

Under the proposed system, up to 75 million litres of water a day could be replaced with treated wastewater during drought conditions. The treated wastewater would be pumped through an underground pipeline from a nearby sewage treatment works and returned to the river.

Thames Water has said the project would not deteriorate water quality and would have to comply with environmental and water quality standards established by the Environment Agency.

The company has also stated that the proposed treated water outfall would be downstream of the designated bathing water site.

Campaigners remain concerned about the potential impact of wastewater infrastructure on river health. Solar has called for stricter wastewater treatment requirements, including additional treatment designed to remove micropollutants.

What happens next?

The immediate priority is to establish how widespread the algae bloom is and whether conditions remain unsafe for recreational use.

The Environment Agency is expected to analyse additional water samples before authorities can assess the full extent of the problem. In the meantime, the advice against bathing remains in place.

For anyone visiting the River Thames near Ham, Kingston or Hampton, the safest approach is to follow official warnings and avoid entering the water while the notice remains active.

The discovery also comes at an important moment for London's ambitions to improve access to the Thames for swimming and other recreational activities. The designation of the city's first official bathing water site was intended to provide a recognised location where water quality could be monitored during the bathing season.

The current algae warning demonstrates that official bathing status does not mean the water will always be safe. Conditions in rivers can change rapidly because of weather, pollution and other environmental factors.

As testing continues, residents and visitors should monitor official water quality updates before planning to swim.

Source: BBC News and Local Democracy Reporting Service (LDRS), published on 19 August 2026.

Disclaimer: This article is intended for general information only. Water conditions can change, so readers should rely on current advice from the Environment Agency, local authorities and official bathing water services before entering the River Thames.