Almost two dozen guns were lost or stolen in London last year, new figures show.
The Gun Control Network said the figures, which show hundreds of guns are lost and stolen nationally each year, are "appalling" and criticised owners who lose their weapons.
The Home Office said the UK has some of the toughest gun laws in the world and that enforcement agencies "will work quickly to crack down on criminals who illegally obtain firearms".
Its figures show 20 guns were lost or stolen in London in the year to March – down from 27 the year before.
Of them, seven were stolen and 13 were lost.
Some 17 of these were shotguns, while one was a rifle.
Across England and Wales, 439 firearms and shotguns were reported lost or stolen in the year to March, a 20% rise from 365 the previous year – of these, 165 were lost and 274 were stolen, both increases from the year before.
While shotguns and rifles accounted for the majority of lost or stolen guns, other types reported missing included handguns, pistols and sound moderators.
Gun owners are required by law to report a lost or stolen weapon within seven days of the incident.
The National Crime Agency said gun crime in the UK continues to be relatively low compared to mainland Europe but recognised "some criminals may try to obtain firearms illegally through theft from lawful licence holders".
A Gun Control Network spokesperson said: "Anybody who loses a weapon is not fit to own a weapon.
"They are actually in breach of the technical requirements of the licence, that it must be kept safe and secure at all times."
A Home Office spokesperson added that the police have powers to revoke a firearm certificate if the holder does not store their weapon securely.
The number of people seeing their licences revoked has also increased across England and Wales.
Last year, 385 firearms licences were removed from owners, a 26% increase from the 306 the year prior, while a further 1,075 shotgun licences were revoked, up by 9% from 982.
Among them were 116 firearm and shotgun licences revoked in London.
Overall, 953 applications for a firearm or shotgun licence from people living in the area were accepted – up from 779 in 2020-21.
It means 23,722 people are now authorised to use guns in the area – of these, 64 are aged 17 and under.
Christian Ashwell, head of firearms threat at the NCA, said: "The NCA works closely with policing partners and engages directly with representatives of the firearms trade and sports shooting associations to promote the safe and responsible ownership and storage of firearms, shotguns and ammunition."