Police officers used a Taser gun on a 12-year-old girl, Merseyside Police has confirmed.
The force said the gun was fired to prevent her "causing serious harm" to herself and others.
Simon Chesterman, of the Association of Chief Police Offices (ACPO), said from what he read about the case the Taser may have saved the girl's life.
However, Mr Chesterman who is Deputy Chief Constable of West Mercia, added it "does sound awful".
Merseyside Police said the girl was shot in July 2011 to "prevent serious harm to both her, the public and the officers", as she was armed with two knives and behaving aggressively towards them.
Mr Chesterman, who is ACPO's lead for armed policing, was commenting after a Freedom of Information (FOI) request revealed the shooting.
He stressed the Taser remained the "quickest, easiest and least injurious" measure of stopping people posing a threat to the public.
"I'm not saying Tasers are risk free... but the risk from the electricity is very low," he said.
He added: "When you hear a 12-year-old being Tasered, it sounds awful. It doesn't mean it wasn't lawful, but it does sound bad.
"From what I've read about that particular case, the individual was intent on self-harming. It may well be that Taser saved her life."
Nearly 13,800 officers across 43 forces in England and Wales are trained to use the weapons - about 10% of the total force.
Taser guns are used to send an electric shock to incapacitate people.