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Sadiq Khan's former policing adviser has joined the Liberal Democrats, saying his children were no longer safe in London due to rising violence.
Leroy Logan, a former police superintendent, said he quit the Labour Party over the London mayor's failure to "grasp" knife crime.
Mr Logan will now become policing adviser to the Lib Dem mayoral candidate Siobhan Benita.
Mr Khan has been approached for comment.
Speaking at the Lib Dem's conference he said: "I've seen my children and their generation grow up in fear.
"It's so tangible. It's been normalised to such an extent it can happen anywhere, not just small pockets of deprived areas."
Mr Logan said the mayor of London "doesn't really understand" knife crime, and has "isolated himself" on the issue.
"He's surrounded himself with people who think they are problem solvers, but are creating more problems on the street because they've lost touch with what is going on."
Mr Logan previously criticised the choice of Lib Peck to run London's Violence Reduction Unit - a role he had also applied for.
Ms Benita, who is running London's 2020 Mayoral election, said: "Sadiq has wasted his mayoral term in not addressing this issue with the urgency it needs.
"While he continues to blame other people, our young children in London continue to be traumatised, petrified and at risk. There is so, so much more we can do."