Speaking at the Royal Naval College at Greenwich, the prime minister made reference to Sunday's knife attack by Sudesh Amman in Streatham that injured three people and November's deadly attack on London Bridge by Usman Khan.
Both men were shot dead by police, and it was later revealed that both were previously convicted of Terrorism Act offences, but released early, as per UK law.
"What we want to do is to make sure that people convicted of Terrorism Act offences are not let out without some process of parole or scrutiny by real experts in the matter, cynical hardened people who can look into their eyes and really think whether or not these people again pose a danger to the public", Johnson said in response to a journalist's question whether he could guarantee that those convicted of terrorist crimes would not be released early.
The prime minister stated that current UK law stipulating that convicts are eligible for early release halfway through their sentence must be changed.
"The law as it stands at the moment demands automatic release, unscrutinized release, and that, I think and most members of the public would agree, is not the right way forward", Johnson added.
The prime minister stated that Justice Secretary Robert Buckland is set to publish a document outlining the government's proposals to toughen release procedures for those convicted of terrorist offences. However, lawmakers may face issues in changing the law to apply retroactively to the estimated 200 people in UK prisons convicted of terrorist offences who will be eligible for early release.
"That is a very complex legal question and as you know, we're bringing forward legislation to stop the system of automatic early release, but the difficulty is how to apply that … retrospectively to the cohort of people who currently qualify", Johnson added.
On Sunday, Sudesh Amman stabbed several pedestrians on Streatham High Road, south London. Government sources reportedly confirmed that Amman had been convicted of 13 terrorist offences. He was released days ago after serving half of his three-year sentence.
Amman's attack follows that of Usman Khan on 29 November, who stabbed five people on London Bridge in central London. Jack Merritt and Saskia Jones, two prison advocacy workers, were killed by Khan, who served eight years of a 16-year sentence before being released.