Khan said Johnson had been 'asleep at the wheel', after ignoring repeated suggestions that the initiative was doomed to fail.
The London Rental Standard was announced four years ago with the target of signing up 100,000 landlords by 2016.
City Hall documentation shows that the scheme has in fact accredited fewer than 2,000 landlords since inception.
Johnson apparently ignored a claim that it would take more than 50 years to accredit enough landlords to meet the target, which was also described as 'clearly unrealistic and unachievable' in internal emails.
The former Mayor was also informed that there were not enough resources to proactively enforce the London Rental Standard.
Speaking at a visit to the Hackney Family Service on Tuesday (May 31), Khan took the opportunity to outline his plans for London's Private Rented Sector (PRS).
He said he wants to extend existing landlord licensing schemes and name and shame 'rogue landlords'.
"I am determined to get a grip on the private rented sector," he said.
"I've met families who have had to move nearly a dozen times with their children because they can't afford the rent or they have been treated terribly by their landlord. This is not good enough."
Khan said his predecessor made little or no progress when it comes to the PRS and that he is determined to stop 'rogue landlords' who 'exploit' renters.
The London Rental Standard, now regarded as a failure, inherited around 13,300 landlords from previous schemes and as of March 2016 had only accredited 1,845 new landlords.
The initiative was also keen to sign up letting agents, with over 300 reported to have signed up this time last year.
Article courtesy of Letting Agent Today | Sign up for Letting Agent Today newsletter | Get this news on YOUR site!