The Queen has said her “thoughts, prayers, and deepest sympathy” are with all those who have been affected by Wednesday’s “awful violence”.
In a message sent to Acting Commissioner of the Metropolitan Police Service, Craig Mackey, the monarch also expressed her thanks and admiration to the Met and others who work to protect others.
The monarch was at Buckingham Palace as the attack took place and a few hours later had her weekly meeting with the Prime Minister, where it is likely she was updated in person about the incident.
She said in her message: “Following the shocking events in Westminster, Prince Philip and I are sorry that we will not be able to open the New Scotland Yard building as planned today, for very understandable reasons.
“I look forward to visiting at a later date.
“My thoughts, prayers, and deepest sympathy are with all those who have been affected by yesterday’s awful violence.
“I know I speak for everyone in expressing my enduring thanks and admiration for the members of the Metropolitan Police Service and all who work so selflessly to help and protect others.”
The Queen was due to open the Metropolitan Police’s new headquarters, New Scotland Yard, on Thursday but late on Wednesday night a Buckingham Palace spokeswoman said that in light of events the engagement had been postponed.
Mr Mackey was due to meet the Queen and the Duke of Edinburgh when they arrived at the new building, which is close to scene of the terror attack.
Two members of the public, Aysha Frade and a man in his mid-50s, were killed when the attacker drove a car across Westminster Bridge, mowing down pedestrians.
Armed with a knife, he then stormed into the Parliamentary estate, fatally stabbing 48-year-old Pc Keith Palmer before being shot dead by an armed officer.
Buckingham Palace would not comment about its security arrangements in light of the terrorist incident.
But it was clear that, with the Queen in residence, some precautions had been taken on Wednesday as the gates, patrolled by armed police, were closed.