A spokeswoman for South Western Ambulance Service said the boy hit by an ambulance in Bath is now in a "stable condition".
"He was transferred to Frenchay Hospital because he is suffering from head trauma and the hospital has a specialist major trauma unit," she added.
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An ejector seat manufacturer is to be prosecuted over the death of Red Arrows pilot Sean Cunningham at RAF Scampton in November 2011.
Police said the "active shooter" had since been shot by officers and they were not seeking any other suspects.
Rain will continue spreading in from the southwest, remaining heavy, especially in western areas.