While the likelihood of basement flooding remains relatively low, the Mayor warned that where it does occur, the impact can be serious — with fast-flowing water posing a danger to life and causing significant property damage.
Mayor Sadiq Khan has warned tens of thousands of Londoners living in basement properties to prepare for potential flash flooding, as new data and worsening weather patterns put large parts of the capital at increasing risk.
The alert comes during London Climate Action Week, with City Hall issuing detailed guidance to 56,000 households across the capital on how to prepare for severe weather events. While the likelihood of basement flooding remains relatively low, the Mayor warned that where it does occur, the impact can be serious — with fast-flowing water posing a danger to life and causing significant property damage.
The warning follows a string of extreme weather incidents in London, including the devastating floods of July 2021, when torrential rainfall overwhelmed drainage systems and forced many residents from their homes. In just two hours, some parts of the capital received more than a month’s worth of rain.
According to the latest analysis by the Environment Agency and City Hall, over 500,000 homes and businesses in London are now at high or medium flood risk — a growing threat fuelled by climate change. Flash floods, in particular, are becoming more frequent during summer storms and are difficult to predict due to the speed at which they can develop.
To raise awareness, a City Hall leaflet is being distributed to residents in at-risk basement properties, offering practical advice on how to prepare. The guidance includes checking drains, signing up for Met Office weather alerts, and creating an emergency “flood kit” containing warm clothing, chargers, medication, and important documents.
“This is no longer a distant climate emergency — it is on our doorsteps,” Khan said. “We saw this clearly during the floods of 2021. I want all Londoners, especially those most at risk, to have the information they need to stay safe and protect their homes.”
In addition to the new guidance, the Mayor is backing flood resilience efforts with wider investment in green infrastructure. A £1.8 billion package announced earlier this year will support flood protection and waterway improvement projects, including initiatives under the Flood Ready London programme. This partnership involves the Environment Agency, Transport for London, borough councils, Thames Water, and emergency services.
Other leaders echoed the Mayor’s concerns. Councillor Kieron Williams, speaking for London Councils on transport and environment, called the 2021 floods “devastating” and stressed the importance of coordinated local action to protect residents.
London Fire Brigade’s Assistant Commissioner Pat Goulbourne added that climate change is intensifying the city’s emergency planning challenges. “Being prepared will help minimise damage to property and ensure the wellbeing of the city,” he said.
Charlie Wood from the Environment Agency warned that “surface water flooding can happen anywhere, to anyone,” urging people to check local flood risks and make plans accordingly.
The warning and interventions are part of a wider effort by City Hall during London Climate Action Week, a festival aimed at finding global solutions to the climate crisis. Alongside the flood resilience push, the Mayor announced new green investments, including:
£12 million for a Green Roots Fund to help councils and communities improve natural spaces;
An expanded Green Finance Fund to support clean energy, transport and heating projects;
£2.4 million for the Breathe London programme, which will monitor air quality in real time via 132 sensors across the city;
A new City Hall solar farm project in partnership with TfL;
The launch of a Climate Finance Taskforce, aimed at unlocking billions in private sector investment to reach net zero by 2030.
Londoners can now access a new resilience toolkit, created by City Hall, to help communities build local flood defences and emergency plans. The toolkit is available at communitiesprepared.org.uk/london-community-resilience-toolkit, while area-specific flood risk data can be found on the Environment Agency’s website.
As the capital faces a hotter, wetter future, officials say early action and awareness are key to minimising disruption and protecting vulnerable households.
