BBC home affairs correspondent Daniel Sandford describes the man named as the attacker - Khalid Masood - as being "quite familiar to police" with previous criminal convictions, but not terrorist-related convictions. 

To that extent, he says the attack did come out of the blue. He adds that yesterday's attack was a breach of the outer perimeter of Westminster security and questions would be asked about the security of Parliament.

But the immediate focus of the investigation would be on Khalid Masood's immediate associates, he adds. 

Home Secretary Amber Rudd warns against pointing any "finger of blame" at intelligence services after the London attack.

Following the terror attack in London on Wednesday, former Government Minister Peter Hain talks about the practical and philosophical challenges of protecting Parliament in a free society.

BBC News is at Trafalgar Square in London where people are gathering to hold a vigil for victims of the Westminster attack at 18:00 GMT.   

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Canadian Prime Minister Justin Trudeau has spoken of a comparable terror attack in Canada during a phone conversation with Theresa May.

In October 2014, a lone gunman stormed into Canada's parliament and  a soldier was killed .

Trudeau said the pair talked about the "resolve" that follows terrorist incidents.

"The people in the UK stand strong in the face of this attack, as we saw in Canada," he said.

A Polish man who suffered minor injuries in Wednesday's attack was released from hospital today, his country's embassy in London says.

World leaders declare solidarity with the UK following Wednesday's attack in Westminster.

Image source, Getty Images

The Met's decision to retire PC Palmer's collar number - 4157U - comes alongside tributes for the killed policeman from colleagues within the force.  

PC James Aitkenhead, who worked alongside PC Keith Palmer in the Territorial Support Group, said he "knew it would be him" when he learned of the incident at Westminster.

"That's just the sort of guy he was, to step straight in when others might step back," he said.

Insp Mark Turner, who also worked with PC Palmer, said he was a "solid, reliable member of the team".

Tom Pursglove, the Conservative MP for Corby and East Northants, has  posted a statement on his Facebook page, external  about yesterday's attack.

It reads: "I can confirm that I was witness to part of the attack and saw, at close proximity, the assailant being shot by a police officer, in what was a great act of bravery in order to protect the lives of others.

"This was obviously a very unpleasant and upsetting experience for all those who bore witness to it and I was particularly grateful to the prime minister who took the time to call me last night - this was extremely kind of her, given all that was going on."

He added: "I wanted to clarify my position and will be making no further comment - not least because of ongoing Police enquiries into what happened yesterday." 

Prime Minister Theresa May has paid a private visit to victims of the Westminster terror attack in hospital, Downing Street says.

She spent 40 minutes talking to those injured in Wednesday's attack and thanked hospital staff for their work.

No 10 did not give further details, stressing it was a "private visit".

Matthew Price reports from Westminster Bridge on the deadly sequence of events.

PC Keith Palmer, who was killed yesterday, was a "friend to everyone who knew him", his family have said in a statement.

Image source, Getty

They described him as "dedicated to his job and proud to be a police officer, brave and courageous."  

"He will be deeply missed. We love him so much. 

"His friends and family are shocked and devastated by his loss and ask that they are left to grieve alone in peace." 

PC Palmer, who was 48 and joined the Met in November 2001, was married with a five-year-old daughter.

Matthew Price reports from Westminster Bridge on how the deadly attack unfolded.

St Mary's hospital in Paddington, central London has said it received a number of people involved in Wednesday's attack.

Dr Julian Redhead, medical director at Imperial College Healthcare NHS Trust, said: "We can confirm that St Mary's Hospital received eight casualties yesterday from the major incident in Westminster. 

"The hospital continues to run smoothly and we are very grateful to all our staff and those of the emergency services for their fantastic response." 

Image source, Tyler Chapperley-Russell and Will Robins

A-level students Tyler Chapperley-Russell and Will Robins met PC Keith Palmer while on a college trip to London last year.

They were taking photos around the Palace of Westminster when they noticed PC Palmer was behind them and he joined in.

They described the Met officer as ''highly professional'' and ''not your typical police officer''.