The Great London Swim will be moved to Millwall Dock for the event on August 31, after tests at the Royal Victoria Dock found the water to be unsafe for immersion sports.
Alex Jackson event director of the Great Swim Series said: “The change in water quality has been unprecedented.
“The safety and comfort of our swimmers is our number one priority, and to ensure we can still provide them with a great open water experience in London, we are moving the event to the nearby Millwall Dock, which has always been on standby as a suitable alternative venue.”
The event will now take place at Docklands Sailing & Watersports Centre at Millwall Dock.
London Mayor Boris Johnson posed for photos as a new scheme was launched, which has brought mobile swimming pools to local communities at 18 London venues.
Speaking at Maria Fidelis School in Camden, north London, Mr Johnson said: "The Make a Splash campaign has seen kids of all ages taking the plunge and trying a sport that offers one of the best forms of exercise known to man.
"As we continue to build on the momentum of last year's Olympic and Paralympic Games, I'm delighted to confirm that the mobile pools will be making yet another tour of duty across the capital so that more people can get the opportunity to take a dip."
Anyone who is interested having a mobile pool to come to your school or community facility can go online here.