"In the UK alone since the Westminster attack in March last year, with the police, we've thwarted a further 12 Islamist terror plots. Twelve occasions where we have good reason to believe that terrorist attack would otherwise have taken place in our country," Parker said, as aired by ITV News broadcaster.
The MI5 chief stressed that the "sustained international focus" was required to defeat the Daesh* terrorist group, which has claimed multiple attacks in Europe and elsewhere in the last few years, even though the group has suffered significant losses in the Middle East.
In December, Parker said that nine terrorist attacks had been foiled in the United Kingdom in the last 12 months.
Two other attacks took place in London in June, both of them involving vehicles that rammed into passers-by.
In September, a bomb exploded in a London tube train. The explosion has occurred at the Parsons Green tube station leaving dozens injured.
*Daesh (also known as ISIS/ISIL/IS) is a terrorist group banned in Russia