Professor George Kassimeris, Professor of Security Studies at the University of Wolverhampton, has been researching and writing on terrorism and political violence for more than 20 years.
He said a terrorist attack by an individual such as this was the hardest kind to prevent:
"Political extremists tend to imitate each other and after the Berlin attack just before Christmas it was, for London, more a question of when rather than if.
"Stopping an individual terrorist attack is the hardest attack to stop let alone prevent. But it is important to remember all those many attacks that never materialised because of the vigilance and prevention strategies of UK's counter-terrorism forces. Today's attack traumatised all of us living in the UK but we should not let allow it to poison and divide us."