Boris Johnson has told the BBC that 74 people jailed for terror offences and released early will have their licence conditions reviewed.
The Ministry of Justice launched the urgent review after convicted terrorist Usman Khan, who had served half of his sentence, killed two people in a knife attack at London Bridge on Friday.
The prime minister claimed scrapping early release would have stopped him.
But Labour is blaming budget cuts for “missed chances to intervene”.
Friday’s attack was brought to an end when police shot Khan dead.
The 28-year-old had previously been jailed over a plot to bomb the London Stock Exchange in 2012.
He was sentenced to indeterminate detention for “public protection” with a minimum jail term of eight years.
This sentence would have allowed him to be kept in prison beyond the minimum term.
But in 2013, the Court of Appeal quashed the sentence, replacing it with a 16-year-fixed term of which Khan should serve half in prison.
He was released on licence in December 2018 – subject to an “extensive list of licence conditions”, Met Police Assistant Commissioner Neil Basu said.
The prime minister told the BBC’s Andrew Marr show it was “repulsive” that someone as “dangerous” as Khan could be released from prison after “only serving eight years”.
He blamed Khan’s release on legislation introduced under “a leftie government”, insisting the automatic release scheme was introduced by Labour – but was challenged about what the Conservatives had done to change the law over the past 10 years in government.
“I’m a new prime minister,” said Mr Johnson. “We take a different approach.”
He added: “I opposed [automatic release] both in 2003 and 2008, and now that I am prime minister I’m going to take steps to make sure that people are not released early when they commit… serious sexual, violent or terrorist offences.
“I absolutely deplore that fact that this man was out on the streets… and we are going to take action against it.”
Mr Johnson said there are “probably about 74 people” who had been subject to early release following serious offences, adding that action had been taken immediately following London Bridge attack “to ensure there is no threat to the public”.
The Ministry of Justice confirmed the 74 figure following the interview.
How the law on early release changed
2003 – The Criminal Justice Act meant most offenders would be automatically released halfway through sentences, but the most “dangerous” would have their cases looked at by a Parole Board. Sentences with no fixed end point, called Imprisonment for Public Protection (IPP), were also introduced.
2008 – Criminal Justice and Immigration Act removed review process by Parole Boards, meaning more offenders were released automatically halfway through sentences. Judges could still hand down life sentences or IPPs for dangerous offenders.
2012 – Usman Khan was handed a sentence with no fixed end date because of the risk he posed to the public. In the same year, the Legal Aid, Sentencing and Punishment of Offenders Act scrapped IPPs and reintroduced the role of the Parole Board for extended sentences of 10 years or more – this time after two-thirds of the sentence has passed. But that did not mean those already serving IPPs would have them lifted.
2013 – During an appeal, Lord Justice Leveson ruled that Khan’s indeterminate sentence should be substituted for an extended sentence with automatic release at the halfway point.
At an event in York, Jeremy Corbyn called for an inquiry into “everything surrounding” Khan, including his sentence and what happened to him in prison.
But he warned against “knee-jerk legislation”, saying the country could “pay a price later”.
In his speech, the Labour leader said: “No government can prevent every attack. No-one would believe any political leader who said they could.
“But the government can act to make such acts of terror less, rather than more likely.”
Mr Corbyn said there needed to be more funding for public services, including probation and mental health, as when they are cut “they leave behind gaps”.
He added: “That can lead to missed chances to intervene in the lives of people who go on to commit inexcusable acts, whether it’s during their childhood, their first brush with the law, their first conviction or in prison through rehabilitation programmes.
“You can’t keep people safe on the cheap.”
Mr Corbyn told Sky’s Sophy Ridge programme terrorists should “not necessarily” serve their full sentences automatically, but that it “depends on circumstances”.
“There has to be an examination of how our prison services work and crucially what happens when they are released from prison,” he said.
Both parties have been accused of politicising the attack.
Liberal Democrat deputy leader Ed Davey told Sophy Ridge on Sunday that he was “alarmed” at Mr Johnson’s reaction to the London Bridge attack.
“In the middle of an election, we shouldn’t be making political capital out of a tragedy, and he’s doing that, and he’s doing that in a way which is misleading people about what the law actually says.”
But Brexit Party leader Nigel Farage tweeted that those convicted of terror offences “should never be released”.
“It is appalling that 74 convicted terrorists are on our streets,” he added. “These are not ordinary criminals but people with a jihadi virus.”
The father of Jack Merritt, one of two victims who died in the attack, said in a now-deleted tweet that his son “would not wish his death to be used as the pretext for more draconian sentences or for detaining people unnecessarily”.
Foreign secretary Dominic Raab said David Merritt should be listened to, declaring “nobody wants to see the politicisation of this”.
But he added: “The question is, who is going to make sure that the overriding priority is avoiding any unnecessary risk to the public?”
“I think if you look at what we’re saying on sentencing… it is the Conservatives who are saying we will stop at nothing to keep people safe.”
Khan was living in Stafford and wearing a GPS police tag when he launched his attack inside Fishmongers’ Hall, where he was one of dozens of students and offenders attending a conference hosted by Learning Together, a prisoners’ rehabilitation programme.
The attack then continued onto London Bridge itself.
Mr Merritt, a course co-ordinator for Learning Together, was one of two people fatally stabbed. A woman who also died has not yet been named.
Dr Vin Diwakar, medical director for the NHS in London, said two victims remain in a stable condition in hospital, while a third has gone home.
Mr Basu said officers had been working “flat out” to try to establish the “full circumstances” of the stabbing.