An AK-47 assault rifle as been found concealed in undergrowth in Webber Row near Waterloo.
The firearm was removed for forensic testing. A Scotland Yard spokesman confirmed that it has been established that the weapon had been decommissioned.
"Firearms – whether decommissioned or not – have the potential to cause fear and harm and I am glad this weapon is now off the streets," said Acting Detective Superintendent Nicola Duffy of Southwark Police .
"I would urge anyone who has information that could assist police in removing firearms from the community to contact us."
Police can be contacted via 101, or in an emergency 999. Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
No-one has been arrested in connection with the discovery of the rifle and officers from Southwark CID are continuing to investigate.