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 been jailed in 2012 over a plot to bomb the London Stock Exchange.
He was given a special jail term known as Imprisonment for Public Protection (IPP), which meant he would serve at least eight years and could not be released unless he had convinced the Parole Board he was no longer a threat.
But in 2013, the Court of Appeal replaced the sentence 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”, police said.
‘Repulsive’
Mr Johnson 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.
“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,” he said.
“I absolutely deplore the that fact that this man was out on the streets… and we are going to take action against it.”
Mr Johnson said there were “probably about 74 people” convicted of serious offences who had been released early – a figure confirmed by the Ministry of Justice.
The prime minister said action had been taken immediately following London Bridge attack “to ensure there is no threat to the public”.
One of the people Khan killed has been named by police as 25-year-old Jack Merritt.
The second victim has not been named, but the University of Cambridge has confirmed she was a former student. One of the three other people injured was a member of staff from the university.
NHS London has said two victims remain in a stable condition in hospital, while a third has been discharged.
They were all attending an event to mark five years of the Learning Together programme – which gives students and inmates the opportunity to study together to help reduce re-offending.
New sentencing
Foreign Secretary Dominic Raab earlier said the Conservative Party had a new policy to enforce a minimum sentence of 14 years for a person convicted of serious terrorist offences.
But Prof Ian Acheson – who advised the government on how to handle extremist prisoners in 2016 – told BBC Radio 4’s The World This Weekend it was not “a question of an arms race on sentencing toughness”, but about what is done when offenders are in custody.
He said 68 of the 69 recommendations he had made around the treatment and risk management of prisoners were agreed by the then Justice Secretary Michael Gove.
But he claimed they had not implemented due to “the merry-go-round of political replacements of secretaries of state”, and the “fairly recalcitrant and unwilling bureaucracy” it created.
Prof Acheson also criticised “crazy failed and ideological austerity cuts” to the police, prison and probation services.
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.
“That can lead to missed chances to intervene in the lives of people who go on to commit inexcusable acts,” he said.
“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”.