A serving member of the British armed forces has been arrested as part of an investigation into "Northern Ireland-related terrorism".
Scotland Yard said the 30-year-old man was arrested in Somerset on "suspicion of being involved in the preparation of acts for terrorism".
He is believed to be from Northern Ireland.
In a statement, police said the arrest was part of an intelligence-led operation involving the Metropolitan Police's Counter Terrorism Command, the PSNI and the South West Counter Terrorism Intelligence Unit.
It said no armed police were involved.
A search operation is under way at an address and at a wooded area in south Devon and at a location in Northern Ireland, which was not given.
It is understood the arrest is linked to two suspected dissident republican arsenals uncovered in forest parks in Northern Ireland earlier this year.
Two separate hauls of weapons were discovered in Carnfunnock and Capanagh parks near Larne, Co Antrim, within three months of each other.
An armour-piercing improvised rocket and two anti-personnel mines were among the cache recovered at Capanagh in May.
Several pipe bombs, magazines and ammunition for an assault rifle as well as bomb component parts and command wires were also concealed in barrels in purpose-built holes in woodland.
In March, bomb-making items were found at nearby Carnfunnock Country Park.
Police said four barrels were unearthed at Carnfunnock - two barrels were empty but two contained a variety of bomb-making components, including wiring, toggle switches, circuit boards, partially constructed timer power units, ball bearings and a small quantity of explosives.
