David Merritt felt heavy with grief when news of the Streatham terrorist attack broke, but he wasn’t shocked. The father of Jack Merritt, a 25-year old prison rehabilitation worker who was killed in the London Bridge terror attack in November, has warned that “the government is failing to do its job to keep the public safe” and that the prime minister’s plans to force through emergency terrorist sentencing is “a hasty measure” that “could be counter-productive.”

Last week, the government announced plans to rush through emergency legislation that would prevent the release of terrorist offenders without parole board-risk assessments. Alex Carlile, the UK’s former reviewer of terror legislation, cast doubt on whether changes could be made retrospectively to the sentences of up to 224 terrorist offenders currently in prison. Ministers expect a legal challenge.

Speaking to the Observer, Merritt was wearied but unsurprised by the government’s “panicky response” and described it as “the austerity chickens coming home to roost”. Just as his son had “devoted his energy to the purpose of prisoner rehabilitation”, Merritt called for urgent and proper funding into prison services to prevent further attacks.

“Keeping people in prison for longer in itself doesn’t keep anybody safe,” he said. “It just means [the government] is kicking the can further down the road: spewing people out further down the line when they’ve been associating with other people of like minds, convincing each other of their radicalisation, that’s obviously not a good thing unless they have received real help to change their ways and there are effective deradicalisation programmes. As it is, from the reports of most prisons, we already know prisoners are locked up for 23 hours a day and there is very little in the way of education and rehabilitation. The resources are not there.”

The Streatham attack, carried out by 20-year-old Sudesh Amman, who had been convicted of terrorist offences and was under police surveillance after being released from prison in January, bore horrific echoes of the attack at London Bridge last year. Amman stabbed three people before being shot dead by police.

“The variety and type of religion these terrorists claim to follow is not what Islam is commonly held to be about, so you have to wonder why someone at the age of 20 has chosen that path,” said Merritt. “It can’t just be to do with somebody preaching to this particular doctrine; it must have something to do with his background, his life experiences, his experiences of living in Britain as an Asian man – what motivates somebody to kill random, innocent people? I don’t understand how people get to that point particularly at that age.”

Response has been varied in the days following the attack. Dame Louise Casey, a former government tsar on integration, called for a “terrorists register” in which offenders would be tracked for life “similar to the one sex offenders have to sign”.

“All terrorists who are convicted of offences, regardless of how serious, would be forced to register for life,” said Casey. “We could then monitor them and intervene in their life regularly. We would know where they were at all times.”

Fiyaz Mughal, founder of Faith Matters and Tell MAMA, called for “younger Muslim chaplains in prisons so that they can tackle the youth of extremism … More than ever, we need to invest in recruiting and training younger Muslim prison chaplains to deradicalise extremists.”

Nazir Afzal, the former chief public prosecutor, recommended a major policy overhaul addressing the root causes of radicalisation. “Look at what the police had to do to the Streatham terrorist. Continuous surveillance from several officers is mightily expensive, and the redirection of even a small proportion of these funds to deradicalisation would be an impressive saving in limited resources, not to mention lives.”

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Merritt agreed. “You can’t overlook the fact that an enormous amount of money has been taken out of the system in the 10 years the Conservatives have been in government. The Ministry of Justice has suffered cuts of 40% since 2010 and you can’t run a prison system by slashing like that. The system is failing.”

While still struggling to process the murder of his son, Merritt said he felt compelled to understand why homegrown terror attacks occured and how they might be prevented. “Belgium seems to have a very effective programme; they have more people returning from fighting in Syria than any other European country and they have managed to contain these sorts of attacks. Why are we not looking at their model of deradicalisation?”

• This article was amended on Sunday 9 February to correct Jack Merritt’s age.