The father of a man killed in the London Bridge terror attack has launched a stinging attack on Boris Johnson, accusing him of using his son’s death to “make political capital” and taking the country “for a ride”.

David Merritt, whose 25-year-old son, Jack, was one of two people killed by convicted terrorist Usman Khan, condemned the prime minister for using the attack as justification for a series of tougher criminal policies.

Posting on his social media account, Mr Merritt responded to a reference of the attack during Friday’s BBC leaders’ debate, saying: “If prisoners have engaged with rehabilitation and turned their lives around, why should they be punished for what Khan did?”

In his first direct attack on Mr Johnson, who he had previously pleaded with to stop politicising the death of his son, he said: “[Jeremy] Corbyn spoke the truth last night. Johnson lied and used our son’s death to make political capital.

He added: “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.”

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

Challenged on the attack during the BBC leaders’ debate – the final head-to-head between Mr Johnson and Jeremy Corbyn – the prime minister said he had “huge sympathy” for both victims’ families. “It was a terrible thing,” he said.

He added: "But I still think it's wrong that someone like Usman Khan who was sentenced 21 years or 16 years plus five on licence should have been out automatically on eight years.”

In response, Mr Merritt, whose son Jack was working at a prisoner rehabilitation conference when he was killed, said: “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 two 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.”

Posting on his Twitter account, he continued: “The prison education, probation and monitoring services have been cut to the bone, and 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."

The Labour leader described the attack last Friday – resulting in the deaths of Jack Merritt and 23-year-old Saskia Jones – as “utterly appalling”.