The five-year-old was airlifted to hospital after the horrific accident in Enfield, north London.
He is said to have suffered a serious chest injury when he was hit with the pellet gun while playing in the garden of his Bullsmoor Lane home at 3.30pm today.
No other children were injured during the incident.
Police said that the boy has been left in a “life threatening condition” as a result of the injury.
A Met spokesperson said: “The child, aged 5, sustained an injury to his chest.
“It appears at this time, the child was playing with other young children in the garden when a pellet gun was loaded and discharged mistakenly by the five year old’s sibling, aged six.
“At this time, the injury is considered to be life threatening”
“London’s Air Ambulance was deployed and have flown the child to an east London hospital.
“At this time, the injury is considered to be life threatening.
“Police remain on scene. No arrests made.”
“Enquiries are ongoing.”