The father of one of the London Bridge terror victims has called Boris Johnson a fraud, accusing the prime minister of making political capital out of the attack.

Writing on Twitter after Friday night's television debate between Mr Johnson and Labour leader Jeremy Corbyn, David Merritt called for an inquiry into the attack, "not a witch hunt".

He wrote: "#LeadershipDebate points missed (or deliberately avoided by Johnson): we don't know all the facts about this case yet, and we won't know for some time - the inquest could take up to 2 years. We don't know why Khan killed, or what, if anything could have been done differently to prevent it.

"Let's have an enquiry, not a witch hunt."

His comments come after his son Jack, 25, died after being stabbed in the chest by Usman Khan during a terror attack on 29 November. Fellow University of Cambridge graduate Saskia Jones, 23, was also murdered.

Image: Saskia Jones and Jack Merritt were killed in the London Bridge attack

The victims were both involved with a prisoner rehabilitation conference where convicted terrorist Khan launched his attack.

In comments shortly after the attack, David Merritt said his son would have been "livid" at how his death has been used to "perpetuate an agenda of hate", amid criticism of the prime minister for politicising the tragedy.

Mr Johnson was condemned for appearing to blame the incident on sentencing laws that allowed Khan to be released halfway through a 16-year jail term for plotting to blow up the London Stock Exchange.

Image: Flowers for victims of the terrorist attack

Despite the criticism, Mr Johnson made similar comments during the Friday night debate.

He said it was "extraordinary and wrong" that Khan had been released halfway through his prison sentence.

Referencing the comments of one of the victim's family, Mr Corbyn said: "What happened on the streets of London was utterly appalling and 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, of course... you must imprison them, of course you try and rehabilitate them if you can."

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David Merritt's comments on Twitter after the debate showed his anger with the prime minister and he urged voters to support Mr Corbyn.

"Secondly, there is no justification AT ALL for cutting the early release tariff from a half to a third of sentences for all sentences over 4 years that carry a maximum life sentence; that's just trying to look tough on the backs of other prisoners' suffering; if prisoners have engaged with rehabilitation & turned their lives around, why should they be punished for what Khan did?

"How will keeping them in for another few years keep public safe if they get released then with the same lack of supervision and support?

"The prison education, probation and monitoring services have been cut to the bone, & overcrowding in prisons is inhumane - prisoners don't win votes unless politicians are promising to be tough on them - but that makes re-offending MORE likely, which makes the public LESS SAFE.

Image: Leanne O'Brien, the girlfriend of Jack Merritt, is comforted by family members during a vigil

"Is that what we want?

"People are being released from prison with nowhere to live, given £47 & told to get on with it. Any guesses how people cope faced with that situation? No prizes. Corbyn spoke the truth last night. Johnson lied & used our son's death to make political capital.

"Wake up Britain: this man is a fraud. He's the worst of us, & he's taking you for a ride.

Image: Jack Merritt was killed in the London Bridge attack. Pic: Facebook

"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.

"For all our sakes, and for the future of the country, please, please do the same."

Sky News has approached the Conservative Party for comment.