The Westminster terrorist was previously known to security services and had been investigated by MI5 over violent extremism.

Prime Minister Theresa May told MPs that the British-born attacker was a “peripheral” figure who was “not part of the current intelligence picture”.

It came as Islamic State claimed that the perpetrator was one of its “soldiers”, while police activity in connection with the investigation has been reported in a number of areas around the country.

The Prime Minister addressed MPs as they gathered at the usual time inside the Palace of Westminster, which a day earlier had come under attack from the knife-wielding terrorist.

POLICE Westminster

An hour after MPs had stood for a minute’s silence in honour of the innocent people killed in the attack, Mrs May delivered a statement with details of the atrocity.

Members of the House of Parliament observe a minutes silence to pay respect to the victims of yesterday's terror attack in WestminsterMembers of the House of Parliament observe a minutes silence to pay respect to the victims of yesterday’s terror attack in Westminster (PA)

Describing the terrorist, Mrs May said: “What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism. He was a peripheral figure.

“The case is historic. He was not part of the current intelligence picture. There was no prior intelligence of his intent or of the plot.”

In a defiant message to a packed House of Commons, Mrs May said: “We will never waver in the face of terrorism.”

Paying tribute to Pc Keith Palmer, who died after being stabbed, she said: “He was every inch a hero and his actions will never be forgotten.”

Pc Keith PalmerPc Keith Palmer (Metropolitan Police/PA)

The identities of the terrorist’s victims on Westminster Bridge have emerged – one a US tourist from Utah celebrating his wedding anniversary, the other a “highly regarded and loved” member of college staff.

Kurt Cochran and his wife, Melissa – on the last day of a trip celebrating their 25th anniversary – were visiting her parents, who are serving as Mormon missionaries in London.

Current terror threat level in the UK

Aysha Frade, who worked in administration at independent sixth-form school DLD College London, in Westminster, is understood to have been 43 and married with two daughters.

A massive inquiry has been launched by intelligence agencies and the police as they attempt to piece together the killer’s movements in the lead-up to the attack.

The address in Hagley Road was raided on Wednesday night (Richard Vernalls/PA)The address in Hagley Road was raided on Wednesday night (Richard Vernalls/PA)

Anti-terror police have arrested eight people in the wake of the attack, and several addresses were raided overnight in London and Birmingham.

At the scene of one of the police raids, a flat in Hagley Road, Birmingham, one witness told the Press Association: “The man from London lived here.”

POLICE Westminster

Surrey Police would not comment on whether there was police activity in their county as part of the probe into the Westminster attack, instead directing queries about the matter to the Metropolitan Police. A Scotland Yard spokesman said the force would not discuss locations “for operational reasons”.

Investigators believe the attacker acted alone, and Mrs May said there was “no reason to believe” further attacks on the public were planned.

Meanwhile, Islamic State made its first public pronouncement since the atrocity. A statement released through the group’s Aamaq news agency said the attacker was a “soldier of the Islamic State”.

Commentators pointed out that the terror group has a record of opportunistically claiming attacks and said it was significant that the statement did not appear to claim it had directed the strike.

A minute’s silence was held nationwide at 9.33am, including in the Palace of Westminster and at New Scotland Yard, to commemorate the three innocent people who were killed.

The timing of the silence was chosen in honour of Pc Palmer’s shoulder number – 933.

Forty other people were injured in the attack, with 29 treated in hospital, where seven remained in a critical condition on Thursday.

POLICE Westminster

The casualties included 12 Britons, three French children, two Romanians, four South Koreans, two Greeks, and one each from Germany, Poland, Ireland, China, Italy and the United States. Three police officers were also hurt, two of them seriously.

Westminster Bridge reopened on Thursday, less than 24 hours after the attack.