Defence Secretary Michael Fallon at Westminster
The defence secretary’s personal security detail dived out of a waiting BMW and ran towards the knifeman, before pulling a handgun and killing him, reports say.
An eyewitness allegedly gave the information to Laura Kuenssberg, the BBC’s political editor.
She told the Daily Politics show: “I understand it was one of the bodyguard of the defence secretary Michael Fallon.
The crucial difference is that those bodyguards are only in the Palace of Westminster when those ministers are actually there."
Sir Michael Fallon is a British Conservative politician and Member of Parliament for Sevenoaks.
Since 2014, he has served as Secretary of State for Defence and been a member of the National Security Council.
Sir Michael Fallon and his security detail
The minister told Sky News Britain is facing a fresh threat.
He said: “This is a new kind of terrorism, this is a new challenge for our security services because these are very ordinary things; a 4×4 vehicle or a knife.
“We’re also dealing with terrorists who don’t want anything, they don’t want to negotiate, they don’t want to take hostages, they’re simply out to kill as many people as possible, completely innocent people, including foreign tourists.”
Sir Michael remained defiant as he continued: “Londoners are taking it on the chin”.
“London is going back to work,” he said. “Coming to Westminster today I passed school children, I’ve heard school trip are going ahead, people are going to their offices and jobs.
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“London has seen terrorism before and London has faced it down and the message today is we will get back to normal as quickly as possible.
“Parliament will be meeting at its normal time today and conducting its normal business was a statement later on from the prime minister.
“I do want to reassure people security agencies are working around the clock now to make sure this attack is investigated and we can be protected against future ones.”
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Security services staff pick through debris, including a car number plate, left following the March 22 terror attack on Westminster Bridge in central London
A spokesman for the Met Police said they are currently not retracting their original statement, which claimed the terrorist was shot dead by a Met Police firearms officer.
The police have "raised a question mark" over the report while insisting errors can be made in the initial confusion.
The Ministry of Defence would not comment.