Police were called to the home of artist Sam Taylor-Johnson and actor husband Aaron after a passer-by spotted a machine gun on a desk inside the house.
The gun, a deactivated M16 assault rifle, had been given to the film-maker and photographer as part of an art project.
A Metropolitan Police spokesman confirmed officers from the firearms unit were called after the gun was spotted through a window of the north London house shortly before midnight last night.
A spokeswoman for the couple, who met when Aaron played John Lennon in her Beatles film Nowhere Boy, said: "We can confirm that early this morning police were called to the London home of Mr and Mrs Taylor-Johnson following a report of a firearm on the premises by a concerned passer-by.
"The family were not present in the property at the time but the police gained entry via a key holder.
"The firearm was located in Sam Taylor-Johnson's basement studio and has been confirmed as a certified and fully decommissioned M16 assault rifle that was provided to Mrs Taylor-Johnson by the international charity Peace One Day as part of an art project entitled 'Peace One Day M16'.
"The project, curated by Jake Chapman, consists of new works by some of the most celebrated names in contemporary art all of whom are transforming fully decommissioned M16 assault rifles."
No arrests have been made.