This article is more than 9 months old

This article is more than 9 months old

Boris Johnson has been accused of twisting the facts of the London Bridge terror attack in a “distasteful” attempt to turn it into an election issue, as he tried to blame Labour for the release of the terrorist who stabbed two people to death.

Despite one of the victims’ families pleading for their son’s death not to be used as an excuse for kneejerk political reaction, Johnson claimed that “a lefty government” was responsible for Usman Khan being freed.

The family of Jack Merritt called for the murder of “our beautiful, talented boy” to not be exploited for political gain, as police named the second victim as Saskia Jones, a prisoner rehabilitation volunteer.

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But the Conservatives nonetheless sought to push a perceived political advantage on the issue, promising to end early release for people convicted of terrorism, while saying Jeremy Corbyn was unable to keep the public safe.

Early on Monday, as the day’s front pages emerged covering a proposed Tory crackdown on those freed after serving sentences for terrorism, Merritt’s father David tweeted saying: “Don’t use my son’s death, and his and his colleague’s photos – to promote your vile propaganda. Jack stood against everything you stand for – hatred, division, ignorance.”

Richard Burgon, the shadow justice secretary, accused Johnson of going “straight from a tragedy to reheating pre-packaged political lines smearing the Labour party”.

The political row came as:

• MI5 began an inquiry into how a suspect whom they had under “active investigation” could have launched such an attack.

• It emerged that investigators discounted Isis’s claim of responsibility and believe Khan acted alone in the planning, which could have started as little as 24 hours before he struck.

• West Midlands police arrested a man who had been convicted of the same bomb plot as Khan on suspicion of preparation of terrorist acts. It followed a review of licence conditions after the London Bridge attack.

• Johnson said there were “probably about 74” convicted terrorists who had now been freed, saying these would be “properly invigilated to make sure there is no threat”.

Khan was released under licence a year ago, seven years into a jail term imposed for taking part in an al-Qaida-inspired bomb plot. On Friday he murdered Jones and Merritt in a knife attack at a conference on prisoner rehabilitation. Khan was tackled by members of the public on London Bridge, and then shot dead by police.

A police statement naming Jones said she had been volunteering at the event organised by Learning Together, a programme run by Cambridge University’s criminology institute. A family statement called her “a funny, kind, positive influence at the centre of many people’s lives”.

Merritt was a course coordinator for Learning Together. In a statement, his family reiterated their call for his murder to not be exploited for political gain.

They said: “We know Jack would not want this terrible, isolated incident to be used as a pretext by the government for introducing even more draconian sentences on prisoners, or for detaining people in prison for longer than necessary.”

The family added: “Jack lived his principles: he believed in redemption and rehabilitation – not revenge – and he always took the side of the underdog.”

Play Video 'We're devastated': Cambridge University and residents react to London Bridge attack – video

On Sunday, Johnson had used a lengthy interview on BBC One’s The Andrew Marr Show to blame Labour for the early release of Khan.

The prime minister said: “His release was necessary under the law because of the automatic early release scheme under which he was sentenced – that was the reality – and that was brought in by Labour with the support of Jeremy Corbyn and the rest of the Labour party.”

He added: “I think it is ridiculous, I think it is repulsive, that individuals as dangerous as this man should be allowed out after serving only eight years and that’s why we are going to change the law.”

The reality of the case is more complex. Khan was jailed in 2012 under an indeterminate public protection (IPP) sentence, a system scrapped under the Conservative-led government the same year. An appeal in 2013 resulted in the sentence being replaced by one of 16 years, and he was freed after serving just under half of that.

Shortly before Johnson’s appearance on The Andrew Marr Show, his staff sent out a lengthy Twitter thread on the PM’s account explaining what they said was the legal context.

This prompted a popular legal blogger and best-selling author, known as the Secret Barrister, to accuse No 10 of having copied and pasted parts of a blogpost they had written on the issue to debunk earlier inaccuracies made by Johnson.

Jo Swinson, the Liberal Democrat leader, condemned Johnson’s response to the attack. She told BBC Radio 5: “You’ve got a community which is coming together in a brilliant way and straight out of the door the prime minister’s trying to make it an election issue – I just think it’s pretty distasteful.”

But the foreign Secretary, Dominic Raab, denied that the Conservatives were politicising the attack.

However, he cited the incident to warn against what he said would be harmful Labour policies. “The question is, for people who are watching the show, who they trust: Boris Johnson to keep us safe or Jeremy Corbyn, with his track record?” he told Sky News.

The Labour leader said on Sunday that convicted terrorists should “not necessarily” serve their full sentence, calling instead for a wider focus on issues such as rehabilitation and deradicalisation

In a later speech, Corbyn condemned cuts to areas such as probation and mental health or youth services, saying this was likely to create “missed chances to intervene in the lives of people who go on to commit inexcusable acts”.

Blaming the Iraq war and other UK foreign policy decisions for fuelling terrorism, Corbyn said: “Real security doesn’t only come from strong laws and intelligence, it comes also from effective public services that have the funding they need.”

Cambridge University’s vice-chancellor, Stephen J Toope, said he was devastated by the attack. He said: “What should have been a joyous opportunity to celebrate the achievements of this unique and socially transformative programme, hosted by our Institute of Criminology, was instead disrupted by an unspeakable criminal act.”

The NHS confirmed on Sunday night that one of the three people injured in the attack had been allowed to return home. The other two remained in a stable condition.