Updated: 5pm
A Royal Marine is being questioned by British detectives investigating Northern Ireland-linked terrorism.
It is understood the arrest is connected to two major dissident republican arms finds.
The 30-year-old serviceman was arrested in a pre-planned swoop in Somerset earlier today.
The operation involved searches at a house and wooded area in south Devon.
In Northern Ireland, officers have also searched a number of properties in Larne, Co Antrim.
The properties are near the two forest parks where the weapons dumps were discovered earlier this year.
The man, who is believed to be from Northern Ireland, was detained on suspicion of preparation for acts of terrorism.
Two separate hauls of weapons were discovered in Carnfunnock and Capanagh parks 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 were discovered in barrels in purpose-built holes in woodland. Also found were bomb component parts and command wires.
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.
These included wiring, toggle switches, circuit boards, partially constructed timer power units, ball bearings and a small quantity of explosives.
After assessing the nature of the weaponry, police concluded they likely belonged to dissident republicans.
Since 2009, dissidents have murdered two soldiers, two policemen and two prison officers in Northern Ireland.
Scotland Yard said the arrest in Somerset was carried out by officers from the Metropolitan Police Service's Counter Terrorism Command.
It was supported by Avon and Somerset and Devon and Cornwall Police.
A Ministry of Defence spokesman said: "We are aware of a police investigation involving the arrest of a member of the Armed Forces under the 2000 Terrorism Act and will assist this investigation fully.
"It would be inappropriate to comment further on an ongoing investigation."
