There has been a sharp rise in the number of grenades seized from criminals trying to smuggle the explosives into the UK, according to government figures.

In the first four months of this year, 17 devices were discovered by UK authorities, compared with 40 seized between 2013 and 2017. The explosives are usually smuggled overland in lorries or underneath cars that arrive by ferry and mainly come from the former Yugoslavia.

Devices have been seized in Sussex and Scotland this year. In the largest haul, Police Scotland recovered six grenades and 1.5kg of dynamite. The spike has prompted concern among investigators that criminals are stashing weapons.

Chris Farrimond, the deputy director of investigations at the National Crime Agency, 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.”

Such devices were used three times in buildings and once against a vehicle between 2013 and 2017.

“They don’t get used very often, but where they have [been used] we have fortunately seen them not used in crowded areas, but they’ve been used quite specifically against either buildings or a vehicles,” said Farrimond. “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.”

The murderer and drug dealer Dale Cregan used grenades as his “calling card” in three of the four killings he carried out in 2012, throwing them at the bodies of his victims after they had been shot.

Farrimond said there were concerns the weapons could get into the hands of terrorists.

Grenades currently have a street value of between about £250 and £750.

Farrimond 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 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.