British Defence Secretary Michael Fallon says the working assumption is that the London attack is linked to Islamic terrorism.
He told BBC Radio Four's Today program: "It was no accident that this attack was on Westminster, because it is at Westminster that we debate differences, very sharp differences, very freely and respectfully between us, and this kind of Islamic terrorism doesn't respect those differences, so it is no accident that there was an attack here."
Fallon said police were investigating whether the attacker acted alone or had help.
"Our forces are working extremely hard to identify those who are involved in this and I have every confidence they will be able to track down the associates of this particular man."
Asked if it was known the attacker had associates, Fallon said: "No. The police have to proceed on the assumption that he may well have been assisted in this task. There may well have been others involved. They have to check all that and they are doing that.
"The working assumption is that this is related to Islamic terrorism in some form. That is their assumption at the moment, but they don't have a full enough picture of this man and his known associates, and who may, or may not, have helped him prepare this attack. That work is still going on."
Fallon praised the police response, saying: "They have been working right through the night, looking into his background, and how he got hold of the vehicle, where the vehicle has been in the last day or two, and who may, or may not, have helped him."
Asked if the attacker was known to police, he said: "I can't confirm that, you will have to ask the police that."
Fallon said a spirit of defiance was the best response to such attacks.
"It is also, I think, evident form everyone involved yesterday that we are not going to let this kind of terrorism win."
