A Watford teenager was arrested after breaking bail conditions while charged with firearm possession.

The 17-year-old, who cannot be identified for legal reasons, is accused of having a "converted blank firing self-loading pistol with a detachable magazine" in London on June 7, this year.

He appeared at Hatfield Magistrates' Court on Monday (August 7) after he breached, or was likely to breach, his bail conditions that day by failing to comply with the conditions of an electronically monitored curfew requiring him to remain at a Watford address during set hours.

The teen denied breaching bail after his arrest, but the charge was proved before he was readmitted to bail with conditions.

The Watford resident is now set for an appearance in Harrow Crown Court on September 7.

He has not yet entered a plea.

The type of firearm possession charge specifies that the "prohibited weapon" was less than 60 centimetres in length.