Reiterating his manifesto promise, Johnson said that funding of £16.5m for an extra 440 police community support officers to patrol buses and Tube stations will come from cutting Transport for London's advertising budget.
The cuts in adspend will mean fewer high-profile campaigns such as the Metropolitan Police's Operation Trident campaign, created by Miles Calcraft Briginshaw Duffy, to fight gun crime.
The Conservative Party has previously criticised the excessive use of public spending on such campaigns.