London: The father of a London Bridge terror attack victim has issued a blistering condemnation of Prime Minister Boris Johnson's election-eve pledge to crack down on convicted jihadists, saying his son would be "livid" that politicians have used his death to "perpetuate an agenda of hate".

In an extraordinary message published on Tuesday morning Australian time, David Merritt blasted the political fallout from the murder of his 25-year-old son Jack and appealed for a rational response to an increasingly fraught debate in the United Kingdom about how to sentence, rehabilitate and release dangerous offenders.

Flowers and tributes left at London Bridge to remember Jack Merritt. Credit:AP

"If Jack could comment on his death - and the tragic incident on Friday - he would be livid. Jack would understand the political timing with visceral clarity," Merritt wrote. "He would be seething at his death, and his life, being used to perpetuate an agenda of hate that he gave his everything fighting against. We should never forget that."

Convicted terrorist Usman Khan stabbed Jack Merritt and 23-year-old Saskia Jones to death at a prisoner rehabilitation conference on Friday. The pair were part of Cambridge University's Learning Together program, in which Khan was taking part.