Boris Johnson has called for an increased use of stop and search powers to combat knife crime.
The Foreign Secretary insisted police had to "come down like a ton of bricks" on gang leaders as he warned against "going soft".
Mr Johnson said when he was mayor of London he adopted a dual approach that boosted stop and search incidents while mentoring young people to prevent them getting sucked into gangs.
We did two things simultaneously, and this is what Sadiq Khan needs to do. You cannot be soft on this. If people are going to go equipped with a knife, they are putting other people at risk and they are putting themselves at risk.
You have got to stop them, you have got to search them and you have got to take the knives out of their possession.
And we did that with Operation Blunt II. We took tens of thousands of knives off the streets. It was controversial, people said it was unfair, but, by God, it worked.
I got serious youth violence down by 32%. We cut the murder rate by 50%.
We got the murder rate down to fewer than 100 a year for several years running, which is amazing for a city of 8.5 million.