Andy Trotter spoke to Channel 4 News after a knifeman mowed down members of the public before killing a policeman at the Houses of Parliament.
Three people were killed along with the officer, who has been named as Keith Palmer.
The attacker was later shot dead, with at least 40 other members of the public sustaining injuries.
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Andy Trotter said the UK would
This is not going to go away, we need to redouble our efforts to protect the country
Mr Trotter said more resources had to go towards the police force with the terror threat “not going to go away”.
He also said he felt the perpetrator was a “self-radicalised” individual.
He said: “It looks very much that at the moment that this could be a self-motivated individual, someone who is self-radicalised because what we’ve seen so far [is] it is a fairly random sort of attack.
London Terror Attack: All the latest pictures Thu, March 23, 2017
5 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
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At least 40 people remain injured
“Despite the terrible deaths today and injuries – it could, of course, have been an awful lot worse [especially] when you look to see what happened in France with the lorry attack last year.”
Mr Trotter said it was impossible to completely guard against those carrying out attacks on crowds with vehicles.
He said: “You can’t guard against that, obviously in the sense that we can protect public places, there’ll be an inquiry into the security obviously at the Palace of Westminster, there’ll be an inquiry into this chap but life will go on.
“The country is full of crowded places. It’s hoping our security services get after these people and do the best they can to thwart it but nevertheless, we are vulnerable, we’ll always be vulnerable and it’s a matter of constantly being alert against that.”
He called for more resources to be pumped into the police force.
Mr Trotter added: “This is not going to go away, we need to redouble our efforts to protect the country.”