Police tasered an 82-year-old man who was then hospitalised for six days.
The pensioner was shot with a 50,000-volt stun gun by armed officers after apparently threatening a man.
They reportedly swooped on him in a west London street before firing the electroshock weapon.
He was later arrested on suspicion of carrying an offensive weapon and criminal damage to a vehicle in Ealing.
However, after complaining he was in pain after arriving to Acton police station, he was rushed to Central Middlesex Hospital by ambulance, where he stayed for six days.
The case is now being investigated by the Independent Police Complaints Commission following the shooting on the morning of June 28.
IPCC Commissioner Rachel Cerfontyne said: “Our investigation will examine whether the use of a Taser in these circumstances was proportionate and appropriate.”
A spokesman for the Metropolitan Police refused to comment while the investigation is being managed by the watchdog.
An 89-year-old man from Llandudno, North Wales, is the oldest man to have been shot by a Taser.
The war veteran was shot by officers after escaping from his care home and threatening to cut his throat with a piece of broken glass.
North Wales Police said they shot the retired carpenter for his own safety in January 2009.
Hailed as a non-lethal alternative to firearms, Tasers, which have a range of around 20ft, have been linked by human rights groups with hundreds of deaths in the U.S. and Canada.
It fires darts attached to lengths of wire which carry an electric charge causing temporary paralysis.
Experts have warned against using them on children and vulnerable adults because of the risk that they can trigger a heart attack.