Gina Partington's 37-year-old son Thomas Dennison was reported missing last month and a body was found in Rusholme, Manchester, three days later.
The 58-year-old mother, from Urmston, Greater Manchester, formally identified the body as that of her son and, following an inquest, the body was cremated on October 30.
But police had actually found Dennison, living rough in Nottingham, four days earlier.
The case has been referred to the Independent Police Complaints Commission to discover why Partington was not told of her son's discovery in the days before she attended the cremation.
Inquiries are continuing to formally establish the identity of the body, but Greater Manchester police said they believe they know who he is.
Next of kin have been informed.
Police said in a statement: "This set of circumstances is clearly distressing and urgent inquiries are ongoing to establish how this happened."
Ref: Reuters