The arrest of a serving British solider in England by detectives investigating Northern Ireland-linked terrorism is understood to be connected to the discovery of two major dissident republican arms dumps.
The 30-year-old was held in Somerset at around 12.20pm on Wednesday and Scotland Yard said a house in South Devon and a wooded area were being searched. Police are also carrying out searches in Northern Ireland.
The man, who is believed to be from Northern Ireland, was detained on suspicion of preparation for acts of terrorism in a pre-planned swoop.
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.