Investigators have seen a sharp rise in the number of grenades seized from criminals trying to smuggle weapons into the UK.
Between January and April this year alone 17 of the devices were seized by United Kingdom authorities compared to 40 that were discovered over the four-year period between 2013 and 2017.
The weapons are mainly from the former Yugoslavia and in the most recent seizures, in Scotland and Sussex, were attached to the bottom of vehicles.
In the largest haul, Police Scotland recovered six grenades and 1.5kg of dynamite.
Chris Farrimond, deputy director of investigations at the National Crime Agency, said the devices are generally smuggled over land on lorries or trailers that arrive by ferry.
While the number of grenades coming into the country appears to be on the rise, the number of the devices exploded remains very low, on average one per year between 2013 and 2017.
This has led investigators to fear that there are stashes of unused weapons in the country in criminal hands.
Mr Farrimond said the devices were mostly used to target buildings or vehicles. He said: “If we just work on the figures that we know about, the ones that have been recovered over the past four years and the ones that we know of that have been exploded, then somewhere, somehow in the UK there are a number of grenades that are in criminal hands and have not been used.”
The devices were used three times in buildings and once against a vehicle between 2013 and 2017.
He continued: “They don’t get used very often but where they have we have fortunately seen them not used in crowded areas, but they’ve been used quite specifically against either buildings or a vehicles.
“Not one of these was actually used against a person, they were used to create fear and or criminal damage.
“It was almost a warning device.”
Multiple murderer and drug dealer Dale Cregan used grenades as his “calling card” in three of the four killings he carried out in 2012, throwing grenades at the bodies of his victims after they had been shot.
Mr Farrimond said there are concerns that the weapons could get into terrorist hands.
A grenade currently costs between around £250 and £750 on the street.
He said: “The bottom line is that firearms do get offered up for sale and so then the question is how accessible is that criminal sale area to somebody who wants to create a terrorist offence of some type.
“Of course there is a concern from our point of view.
“Protecting the public is rightly at the forefront of everyone’s mind.
“Of course we have a concern that they could fall into terrorist hands and they could be used in a particular way.”
Of the 17 seized so far this year, 12 were military and viable; one was improvised, three were imitation or deactivated, and one was real but not viable.
Last month workmen uncovered a grenade while digging up a street in Glasgow. Part of Pollokshaws was cordoned off amid the bomb alert.
In May last year bomb disposal officers removed a shell and hand grenade from a house in Dumfries and Galloway.