The Metropolitan Police collects data on the number of homicides (murder, manslaughter and infanticide) and not murder specifically. We assume Johnson is referring to homicide statistics.
Official figures show the number of homicides fell below 100 once during Johnson’s tenure as mayor of London, to 95 in 2014. In his last full year in office, 2015, it was 120.
If Johnson has access to non-public data that is specifically about murders rather than homicides, he should say so, so that his claim can be verified. When Full Fact checked this claim in 2018, it asked Mr Johnson’s office for more information about the source of his figures, but never received a reply.
Boris Johnson’s statement was false (unless it relied on non-public data that can’t be checked). I asked the prime minister what evidence he had to support his claim made on the floor of the House of Commons. I also asked if he had plans to correct the record, as the ministerial code demands. I am awaiting a reply.