The justice secretary has insisted that the government is not trying to exploit the deaths of two young people in a terrorist attack for election purposes after the father of one of the victims criticised the “ignorance” of the political response.

Robert Buckland denied that cuts to prison officer numbers and probation services might have played a role in increasing the risk from terrorism, and said “public protection” had to be at the centre of any response to the deaths on Friday of Jack Merritt and Saskia Jones.

But Buckland faced criticism from Phillip Lee, the former Conservative justice minister who defected to the Liberal Democrats in September. Lee said the response had to be based around investment in courts and probation, accusing Johnson of instead using “the Trump playbook”.

Buckland reiterated the government’s stance that Labour was ultimately to blame for the release of Usman Khan, who carried out the stabbings, as his early release last year came under sentencing guidelines passed by the last Labour government. Labour called the idea “ludicrous”.

Merritt’s father expressed outrage at newspaper coverage of Boris Johnson’s pledge to respond to the attack with a “blitz” against terrorists freed from jail. In a tweet, David Merritt hit out at what he called “vile propaganda”, saying: “Jack stood against everything you stand for – hatred, division, ignorance.”

David Merritt (@butwhatifitsall) 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. https://t.co/R8LO16lugk

Khan was released under licence seven years into a jail term imposed for taking part in an al-Qaida-inspired bomb plot.

At a conference on prisoner rehabilitation he killed Merritt, 25, a course coordinator for Learning Together, a programme run by Cambridge University’s criminology institute, and Jones, 23, who was volunteering at the event. Khan was tackled by members of the public on London Bridge and shot dead by police.

Asked if he was “a little ashamed” about the use of the death of two young people for electoral purposes”, Buckland told BBC Radio 4’s Today programme: “I do think we need to pause and get the tone of this debate right.”

He added: “But, public protection has to be at the heart of the duty of any government, and I have to put that first and foremost, when considering first of all existing offenders and secondly the future sentencing regime for terrorists.

“We’ve got to get it right and there are questions to be asked about why automatic early release was being deployed.”

Asked about the government’s crackdown on 70 or so other people convicted of terrorism offences and released on licence, Buckland said these were being checked to ensure they were keeping to licence conditions, and that these were sufficiently strict.

Asked whether he should take into account the victims’ views on rehabilitation, Buckland, a former criminal barrister, said terrorism suspects were “a particularly difficult and complex cohort of people”.

On rehabilitating criminals generally, he said: “I have met some who have been capable of rehabilitation and who have managed to move away from crime. But I have met others who have not. And the difficulty here is working out who is capable, and who isn’t.”

Saying Labour did not appreciate “the importance of public protection when it comes to offences of terrorism”, Buckland denied that cuts to court, prison and probation services had a role in what had happened.

But speaking to Sky News, Lee said Johnson was “just pressing buttons” on the subject, saying: ”We know this is the Trump playbook.”

Lee said: “The practical reality is it’s extremely difficult to do that if you haven’t got the prison places. And we never really had enough prison places in all the time I was a justice minister.

“You can address this by investing in it, investing in the courts, particularly investing in probation, because it’s a cycle – from police, courts, probation. They’ve all got to be working well if you want to reduce crime, reduce pressure on prison, and be able to lock up the people like Usman Khan for much longer, indeed for ever.”

Asked about the Conservative response to the attack, Andy McDonald, the shadow transport secretary, said politicians should listen to David Merritt: “If we’re going to open this up, I’m not sure this is the right moment.”

He condemned Tory attempts to blame Labour for Khan’s release, saying: “So it’s perfectly proper to attack Tony Blair, who hasn’t been prime minister since 2007? It’s absolutely ludicrous. They’ve had 10 years to address this.”