Police found the bodies of the four-year-old boy, his father, a 44-year-old engineer, and his 27-year-old mother in the bedroom of their apartment where they are thought to have been for several days.

Officers also found a letter from the parents explaining they could not bear to see their cherished son with such a severe disability, police said.

Neighbours alerted police in Derendorf, western Germany, after not seeing the family for some time.

Prosecutors in Düsseldorf, who are leading the inquiry, said the couple had killed their son before taking their own lives.