Richmond Council has issued the warning after groups of young people, many under the influence alcohol, have been seen on the banks of the Thames.
Teddington Lock is a particular area of concern; a 12 year old needed medical help after getting into difficulty last week.
The council says it, the police and fire brigade are becoming increasingly concerned.
People are being urged to follows the RNLI’s drowning prevention guidance and “Respect the Water.”
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