A row has broken out between the Conservatives and Labour over the practice of reducing jail terms for violent offenders following the London Bridge terrorist attack.

The home secretary, Priti Patel, blamed the previous Labour government after the party’s MP Yvette Cooper asked how the attacker, Usman Khan, could have been released when he was deemed dangerous.

Khan was jailed for terror offences in 2012. He was released from prison on licence in December 2018 after his initial sentence was quashed on appeal.

The Parole Board said it had no involvement in his release, saying Khan “appears to have been released automatically on licence (as required by law)”.

In a series of tweets on Saturday, Cooper wrote: “Usman Khan was sentenced for serious terror offence in Feb 2012. Thought to be so dangerous by judge he was given IPP sentence to prevent release if still serious threat.

“Instead he was released 6 yrs later without Parole Board assessment. How [could] this be allowed to happen?”

Patel responded on Twitter: “Because legislation brought in by your government in 2008 meant that dangerous terrorists had to automatically be released after half of their jail term.

“Conservatives changed the law in 2012 to end your automatic release policy but Khan was convicted before this.”

Usman Khan was sentenced for serious terror offence in Feb 2012. Thought to be so dangerous by judge he was given IPP sentence to prevent release if still serious threat. Instead he was released 6 yrs later without Parole Board assessment. How cd this be allowed to happen? ...1 — Yvette Cooper (@YvetteCooperMP) November 30, 2019

Boris Johnson, speaking at the scene of the attack on Saturday said cutting jail sentences in half and letting violent offenders out early “simply isn’t working”.

The prime minister has pledged to “toughen up sentences”, while Labour leader Jeremy Corbyn said urgent questions needed to be answered about the role of the Parole Board and probation services.

Johnson said: “I’ve said for a long time now, that I think the practice of automatic, early release where we cut a sentence in half and let really serious and violent offenders out early, simply isn’t working.

“And I think you’ve had some very good evidence of how that isn’t working, I’m afraid, with this case.”

The Parole Board could not be involved in this decision @jeremycorbyn. Your party changed the law in 2008 so that Khan was automatically released irrespective of the danger he posed. Very concerning that you want to be PM but don’t understand this.https://t.co/uqUBggmEbg — Priti Patel (@patel4witham) November 30, 2019

The Metropolitan police assistant commissioner Neil Basu said Khan was subject to an “extensive list of licence conditions” on his release from prison and that “to the best of my knowledge he was complying”.

Corbyn called the London Bridge attack “a complete disaster”, adding: “I think there is also a question about what the Probation Office were doing – where they involved at all - and whether the Parole Board should have been involved in deciding whether or not he should have been allowed to be released from prison in the first place, and also what happened in prison?

“That somebody who clearly was a danger to society … was he given a deradicalisation programme or not?

“I think we have to ensure that the public are safe, that means supervision of prisoners in prison but it also means supervision of ex-prisoners when they are released ahead of the completion of their sentence, to have tough supervision of them to make sure this kind of danger is not played out on the public in future.”

In 2012, Khan was sentenced to indeterminate detention for “public protection” with a minimum jail term of eight years after being convicted for his role in a plot to bomb the London Stock Exchange.

The sentence would have allowed him to be kept in prison beyond the minimum term. However, the court of appeal quashed the sentence in 2013, replacing it with a 16-year-fixed term, of which Khan should serve half in prison.

He was released on licence last year but was required to wear an electronic tag while he lived in Stafford.

Khan’s solicitor, Vajahat Sharif, said his client had asked for help to be deradicalised while in prison.

Sharif said: “He requested intervention by a deradicaliser when he was in prison.

“The only option was the probation service and they cannot deal with these offenders.

“He asked me on the phone to get assistance from a specific deradicaliser.

“Probation do a good job with conventional offenders but they can’t deal with ideological offenders.”

The shadow home secretary, Diane Abbott, wrote on Twitter that the government needed “to listen to police chiefs”.

“We need the proper resources to monitor convicted terrorists.”

Sentences of Imprisonment for Public Protection (IPPs) were introduced in 2005 and given to violent or sexual offenders who posed a risk to society.

They would not include a fixed term of imprisonment but instead would give an offender a minimum period to serve before they could be released.

A prisoner serving an indeterminate sentence would stay in prison until it was found they were safe to be released. IPPS were abolished in 2012 but not for existing prisoners.

Khan and two others were originally given indeterminate sentences with a minimum term of eight years behind bars instead of a fixed term.

Khan, along with Nazam Hussain and Mohammed Shahjahan, appealed against their sentences and had the indeterminate sentences dropped by the Court of Appeal in 2013.

Mr Justice Leveson found the original decision had “wrongly characterised” the three men as more dangerous than the other defendants.

The former chief prosecutor, Nazir Afzal, said the government was repeatedly warned of the risk posed by convicted terrorists being released from prison while still radicalised.

Writing on Twitter, he said he had spoken to Johnson in 2016 at Brunel University, in Uxbridge, west London, where Johnson is an MP.

Afzal said: “He asked me what keeps me awake at night and I told him it was this issue.

“When he wanted to know what to do about it, I told him it was more resources for one-to-one deradicalisation.

“Back then, he hadn’t found the ‘money tree’ so he frustratingly said there was no money [...].

“The problem of those convicted for terrorist-related offences being released from prison whilst ostensibly rehabilitated but still radicalised was one that many of us raised in meetings with this government over the past few years.”