After the London Bridge attack, Prime Minister Theresa May said that Britain was under threat from a new breed of crude copycat militants.
The Scotland Yard has confirmed the arrest, which brings the total to 21 arrests in the case.
"Getting hold of a 7.5 ton lorry - the effects could have been even worse", said head of London's Counter Terrorism Command Dean Haydon.
The belts were still attached to the men when they were killed, Metropolitan Police said.
He was arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.More news: GOP Lawmaker Scalise Still Critical Though Doctors `Encouraged'
Using these knives they stabbed five people to death, and injured many more including an unarmed police officer during an eight-minute rampage that began with their using a hired van to drive into pedestrians on London Bridge.
Dean Haydon, the police commander leading the London attack investigation, said: "I have not seen this tactic in the United Kingdom before, where terrorists create maximum fear by strapping fake explosives to themselves".
They have released images of the leather belts, which were strapped with disposable plastic water bottles wrapped in silver masking tape.
Commander Haydon has also appealed for information about the pink Ernesto-branded ceramic kitchen knives used by the attackers: "We've been working round the clock to understand what these men did in the lead-up to the attack but we need to know more about these unusual knives".
One of the first police officers on the scene of the London Bridge attack has described the "pandemonium" as people ran for their lives while others lay bleeding on the ground.More news: Worth the Upgrade: The new iPad Pro
"The people I saw had cuts and what I assumed to be stab injuries", he said. Ho said he couldn't attack them as one of them might detonate what he believed was a genuine suicide belt and kill them all.
Police say they have interviewed 262 witnesses, from 19 countries.
Cole, 41, said the attack was "the most challenging, most intense situation I've dealt with" in an 18-year police career.
"I spoke with the 200 frightened people in the basement and told them that there were armed officers outside, we were safe and we would evacuate them as soon as we could and I got a big round of applause", Cole said in the account.More news: Gingrich says presidents can't obstruct justice - but accused Bill
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