LONDON - The British National Crime Agency on Monday warned that London faced a new danger posed by criminals who may try to make guns using 3D printing technology.

The law enforcement organization issued the alert in its annual strategic threat assessment, following the discovery of "component parts for a firearm and a digital guide on how to produce a 3D-printed firearm" in London last year.

The British agency said "completed 3D metal handguns" had been printed in other countries.

Although their effectiveness was "uncertain", technological advances and falling costs threatened to encourage production, it added.

In October 2013, a suspected gun factory which used a state-of-the-art 3D printer was uncovered by the Greater Manchester Police. Officers believe a gang were using the machine to make deadly firearms.

The discovery was believed to be the first of its kind in Britain.