The Acting Commissioner of Scotland Yard was at the scene of the terror attack and witnessed the events unfold, it has emerged.

Craig Mackey, who is in temporary charge of the Metropolitan Police following the retirement of Sir Bernard Hogan-Howe earlier this month, was in Westminster on a routine visit when the incident occurred.

Commander JB Harrington said Mr Mackey was being treated as a "significant witness" to the events and therefore it would be inappropriate for him to make a public statement at this time.

Mr Mackey took charge of the country's largest force earlier this month and is due to hand over to the new Met Commissioner, Cressida Dick on April 1.

A former Chief Constable of Cumbria Police and was in charge of the force in 2010 when gunman Derrick Bird shot dead 12 people.

He joined the Met in 2012 as Deputy Commissioner.

Scotland Yard's new premises are just yards from the scene of the incident and it is not clear whether Mr Mackey was in the Palace of Westminster at the time or in the surrounding area.

Giving a statement outside Scotland Yard, Commander Harrington said: "A senior national coordinator has declared this a terrorist incident.