Five people were killed and about 40 injured in London on Wednesday after a car ploughed into pedestrians and a suspected terrorist stabbed a policeman close to Britain's parliament, Reuters reported.

Among those killed were a policeman and three pedestrians, as well as the attacker.

The three pedestrians killed had been hit by the car as it sped across Westminster Bridge before crashing into railings outside parliament.

Prime Minister Theresa May condemned the attack as "sick and depraved" and said in a statement that “the location of this attack was no accident."

"The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."

Any attempt to defeat those values through violence was "doomed to failure", May said.

Mark Rowley, Britain's most senior counter-terrorism officer, told reporters the attack started when a car was driven over Westminster Bridge, hitting and injuring members of the public and three police officers.

"A car then crashed near to parliament and at least one man, armed with a knife, continued the attack and tried to enter parliament," Rowley said.

He said the police's "fast-paced investigation" was working on the assumption that the attack was "Islamist-related terrorism". Police believed they knew the identity of the attacker but would not provide details at this stage, he said.

Reuters reported that this was the deadliest attack in London since four British Islamists killed 52 commuters and themselves in suicide bombings on the city's transport system in July 2005, in London's worst peacetime attack.

Wednesday’s attack however took place on the first anniversary of attacks by Islamist militants that killed 32 people in Brussels.