An assailant who stabbed a policeman inside the perimeter of Britain's parliament was shot by police, David Lidington, the leader of the House of Commons, told parliament.

"The alleged assailant was shot by armed police, an ambulance is currently attending the scene to remove the casualities," he said.

"There are also reports of further violent incidents in the Palace of Westminster but it would be wrong of me to go into further details before confirmation from the police."