Speaking on the Andrew Marr Show on the BBC, the Prime Minister and the host locked horns on a number of issues during a bad-tempered interview.
The Prime Minister said that the release of Usman Khan was down to law changes made in 2008 under former Labour prime minister Gordon Brown.
Khan, who was sent to prison in 2010 for terror offences, was released in 2018 automatically having served half of his 16-year sentence.
He had initially been given an indeterminate sentence, but this was overruled in 2012 following a judicial review.
In a fiery exchange, Mr Marr pushed the Prime Minister on whether cuts under the Conservative administration that took power in 2010 could have been a mistake, to which Mr Johnson replied: “No.”
He said that the Labour government left the economy “in ribbons” and claimed the Conservatives had “governed sensible and prudently”.
Mr Johnson added: “The reason this killer was out on the streets was because of automatic early release which was brought in by a lefty government.”
Pressed on cuts to prison and probation services and the rising levels of assaults on staff during this time, he added: “That is why this new Conservative administration is putting £2.5 billion into our prison service.”
Asked why this has not happened under the last years of Conservative government, he added: “I’m a new Prime Minister, we take a different approach.”
Earlier on the same programme, Labour peer Baroness Chakrabarti has said it is “unedifying” to talk about “throwing away the keys”.
She said: “I personally think it’s too soon for any kind of blame game.
“What I will say is that supervision in prison and outside of prison is very resource intensive, and 40 per cent cuts do not stand with the idea that we want to monitor people and supervise them better.”
Jeremy Corbyn, the leader of the Labour party, had earlier appeared on Sophy Ridge on Sunday and said that terrorists should “not necessarily” serve the whole sentence handed down to them.
He said: “I think it depends on the circumstances and it depends on the sentence but crucially depends on what they’ve done in prison.
“I think there has to be an examination of how our prison services work and crucially what happens to them on release from prison.”