The powder was reported at Caldecotte Lake on Sunday 16th January.
The RSPCA Milton Keynes and North Bucks Branch has issued a warning on social media.
A dog has fallen ill and was taken to hospital after a trip to Caldecotte Lake. The charity has reported that this was the result of some unknown powder that was found in the lake.
The RSPCA warned: "Please be aware and take care when walking dogs or children in this vicinity."
Some residents have taken to Facebook to suggest that the powder could be markings from a running race, others have questioned whether it was poison.
MKFM has contacted the RSPCA for an update on the substance.