Both houses of the British parliament will sit as usual on Thursday, officials said, following an attack outside the building on Wednesday in which four people died.

"We can confirm that both the House of Commons and the House of Lords will sit tomorrow at their normal times," the parliament Twitter feed said.

At least 20 people were also injured after a car ploughed into pedestrians and an attacker stabbed a policeman outside parliament, in what police called a terrorist incident.