Hundreds of guns and thousands of rounds of live ammunition were seized by police in a dawn raid on a semi-detached suburban house.
Twelve Operation Trident officers entered the four-bedroom property in Dartford, Kent, at 6am and unearthed a cache of pump action shotguns, automatic and semi-automatic weapons.
The Metropolitan Police described it as the biggest weapons haul in the force's history but conceded that some of the guns may be legally registered.
Plain-clothed officers arrested a 55-year-old man at the house on suspicion of supplying firearms from a residential address and took him to a north west London police station.
The property is listed under the name of Michael Shepherd, a registered gun dealer, who runs a website advertising the guns he sells. Mike Yardley, from the Shooting Sports Trust, said Mr Shepherd was well known as a gun dealer and collector.
"He specialises in weapons of the Victorian and Edwardian period and typically holds a very large stock. The man has been operating a website where he has been advertising his wares for a significant period of time."
Mr Yardley described him as a kindly but eccentric man of Asian descent who is a devout Catholic and loves Elvis Presley.
"He is very hospitable but rather an eccentric chap. I have been struck by the extent of his collection which is historically significant. He has one of the largest private collections in the UK - it is enormous and all in a semi-detached house in Dartford."
In the raid, a second home and a business address nearby were searched, with four guns discovered at the house. A "substantial" amount of cash was found at all three premises. A simultaneous operation was carried out in New Jersey, in the United States, and a 40-year-old man was questioned. US authorities said they had been working with the UK investigating international weapons trafficking for the last eight months.
At the house in Dartford, thousands of rounds of ammunition and a number of weapons were removed by police, including M16 rifles and revolvers in see-through plastic bags. Many of the weapons were stored in the garage while inside the two-storey house, antique weapons were mounted on the walls. Officers said they found guns in every room of the £250,000 property, except the bathroom.
