Westminster attacker was British-born and 'previously investigated' by M15

An attack on civilians and police outside the UK parliament was carried out by a British-born man who was previously investigated by domestic intelligence agency M15, Prime Minister Theresa May told the House of Commons on Thursday.

"Some years ago, he was once investigated in relation to concerns about violent extremism. He was a peripheral figure," May told a meeting at the UK parliament.

"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."

May added that the attacker - who killed at least three people including a police officer before being shot dead by police - is believed to have acted alone.

Increased police patrols would take place in London in the coming days "as a precautionary measure" in order to "reassure the public".

London remained largely undisturbed on Thursday despite increased security on the streets.

UK's top anti-terror officer Mark Rowley said that 29 people were being treated in hospital, including seven who remained in critical condition, following the attack.

Wednesday's attack was the deadliest in the UK since four suicide bombers killed 52 people on London's transport system in July 2005.

UK police have also carried out a number of armed raids, including in Birmingham, since the attack leading to the arrest of seven people.

Defence Minister Michael Fallon told BBC radio that Wednesday's carnage was linked to "Islamic terrorism in some form".

London's mayor Sadiq Khan has scheduled a candlelit vigil set to take place in Trafalgar Square beginning at 6pm.