The father of a man killed in the London Bridge attack has accused Boris Johnson of lying and making “political capital” from his son’s death during the BBC leaders’ debate.

David Merritt – whose son Jack was stabbed to death by convicted terrorist Usman Khan – called the prime minister a fraud following his appearance on Friday night’s TV debate, six days before polling day.

Following last Friday’s attack, the Conservatives have pledged to toughen up prison sentences. Merritt believes this would have angered his son, who had “devoted his energy to the purpose” of prisoner rehabilitation.

“Johnson lied and used our son’s death to make political capital,” Merritt said in a thread posted on Twitter on Saturday. “Wake up Britain: this man is a fraud. He’s the worst of us, and he’s taking you for a ride. You may think the options open to you in this election are not entirely to your liking. Me neither, but I’ll be voting least worst option: anti-Tory.”

During the debate on Friday, the Labour leader, Jeremy Corbyn, challenged Johnson over remarks Merritt made last week, criticising the use of his son’s death as a pretext to introduce harsher criminal justice reforms.

Corbyn said: “I was very moved by what Jack Merritt’s father said about what his son was trying to do. That he wanted a society where you did address the huge problems where somebody committed awful acts like that – yes of course, you must imprison them, yes of course, you must try and rehabilitate them if you can.”

Responding to Corbyn’s challenge, Johnson said he had “huge sympathy for Mr Merritt and the relatives and families of both victims, and it was an absolutely terrible thing”.

Play Video 2:55 Austerity, racism, the NHS and Brexit: Corbyn and Johnson clash in BBC debate – video highlights

Johnson added: “But I still think it’s wrong that some one like Usman Khan, who was sentenced to 21 years … should have been automatically released on eight years, when the judge, when he was first convicted, made it clear that he was a very serious jihadi.”

The attack took place on 29 November just before 2pm in the Fishmongers’ Hall at an event run by Learning Together, a programme focused on prisoner rehabilitation.

Khan had been released automatically from prison on licence in December last year after serving eight years for plotting a terror attack on the London Stock Exchange.

He had originally been jailed indefinitely in 2012 under an imprisonment for public protection (IPP) sentence. This type of sentence was later scrapped by the Tories and Khan was sentenced to 16 years following a successful appeal in 2013.

Khan was automatically released midway through this sentence under now-scrapped sentencing laws. The parole board did not assess Khan prior to his release, despite the requests of a judge.

On Saturday morning, Merritt said that Corbyn “spoke the truth last night”, adding that the facts of attack – including Khan’s motivation and what could have been done differently – will not be known until an inquest is complete.

He called for an inquiry, rather than a “witch hunt”, to examine what could be done to prevent future attacks and accused the Conservatives of “trying to look tough on the backs of other prisoners’ suffering”.

The row came as the Conservatives placed reducing immigration at the heart of their last-minute pitch to voters, pledging to end “preferential treatment” of EU nationals, reduce numbers overall and trigger a big reduction in low-skilled migrants.

Key to the new Tory system will be ensuring all migrants have a “digital immigration status” after 2022. It will be used to crack down on illegal overstaying, though the party insists it will help legal migrants prove their status.

The new system would divide people into three categories – exceptional talent, skilled workers and “sector-specific” – the latter made up of categories that will change based on advice from the independent Migration Advisory Committee (MAC). It will screen all prospective migrants for previous criminality and bar those with serious convictions.

In a letter to voters, Johnson described the election as one of the most important in more than a century. “In 1906, the Liberal government won a majority and the balance of power crucially tipped towards democracy,” he writes. “In 1945, the Conservative party had lost its way and the Attlee government created the NHS – one of the great achievements in British politics in the 20th century. And in 1979 the Thatcher government dragged Britain out of the nightmare of the 1970s.

“Now, in 2019 we face another of these historic elections. Like 1906, 1945 and 1979 the impact of this election will be felt for decades to come. But unlike those elections, in 2019 it is not a single party that has lost its way, but the entire parliament.”