Mike Haines, whose brother David was murdered by ISIS, has called for unity after the London attack
David Haines, 44, was beheaded in Syria three years ago by the evil group after being held prisoner for 18 months.
His brother Mike, who works with groups to combat extremist indoctrination of young people in the UK, said it was "vital" that communities "remain calm and united” even in the midst of recent events.
Mike said: "My thoughts and prayers go out to the family and friends of the victims of this tragic attack in the heart of our nation's capital.
At least three members of the public were killed in the horrific attack
The attacker drove through crowds on Westminster Bridge before heading to the Houses of Parliament
"In the midst of the hurt and heartache that we are all feeling in this moment, it is vital that we as a society remain calm and united, no matter who or what was behind this attack.”
Police believe they know the identity of the as-yet unnamed suspect, who was killed following his rampage through central London.
London Terror Attack: LIVE PICTURES Wed, March 22, 2017
Two dead, 12 injured as knifeman mows into crowd in Westminster
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
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It is vital that we as a society remain calm and united
Up to five people – including Pc Keith Palmer and the perpetrator – were killed in the horrific incident where more than 40 hurt, with some described as suffering "catastrophic" injuries.
Mike continued: “Terrorism, in all its guises, seeks to spread fear and divide us. In confronting this evil, we must stand shoulder to shoulder, with confidence that we are all stronger when we stand together.
"The reports of kindness and solidarity amongst those on the scene, proves our togetherness.
"Whatever the motivations, they will not succeed in sewing distrust amongst us – I have seen first hand that we are stronger than that."
Police respond to the scenes of the London attack London Terror Attack: Politicians from around the world react Thu, March 23, 2017
Politicians from around the world have responded to the London terror attack which led to the death of five people – including a police officer and the attacker – and injured more than 40.
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At least five people have been killed including a policeman and the attacker
Yorkshire-born David Haines, who was brought up in the Perth area of Scotland, was one of a string of hostages beheaded by ISIS, whose filmed executions involved the notorious terrorist Jihadi John.
The former RAF engineer's name was eventually made public when he appeared in the background of a video showing the execution of Steven Sotloff, an American journalist also seized by terrorists.
Following the recent horrors, terrorism experts have warned the public to be vigilant of further attacks using vehicles.
The threat level in the UK still remains at severe, which has been the case since 2014.