Picture credits: Bobby Dean MP on Instagram.
London Fire Brigade is on scene attempting to contain the spill along with Thames Water on cleaning efforts.
Some wildlife are covered in the oil, particularly in Watermeads Nature Reserve in Mitcham.
The public is being advised not to touch the water or let their dogs in it and it avoid feeding wildlife as that will encourage them to ingest contamination.
A spokesperson for Merton Council said: “The council is aware of a diesel spill from Croydon that has polluted parts of the River Wandle – particularly in Watermeads Nature Reserve, Mitcham – with some wildlife covered in oil.
“The Environment Agency (EA) is onsite leading a multiagency response, of which we are a part.
“The London Fire Brigade has been attempting to contain the spill and Thames Water is working on cleaning efforts.
“The public is advised not to touch the water, nor allow pets into the water.
“In addition, residents should not feed wildlife in the waterway as that will encourage them to ingest contamination.
“An investigation will be led by the EA.
“The council will provide an update when it has more information.”