An affordable housing target cannot be set, Sadiq Khan admitted when asked how his plans to deliver his housing pledges are progressing.
The London Mayor was grilled by Assembly members in a three-hour Mayor's Question Time session, during which the topic of housing was raised.
Labour Assembly Member Tom Copley asked Mr Khan: "Can you outline how through other means you will achieve your target of 50% of new homes being genuinely affordable?"
But in a blow to Londoners, whose priority during the Mayoral elections was housing, he replied, "I cannot set a target for affordable housing."
During his election campaign, which Conservative party opposition Zac Goldsmith had called "a housing election", the Mayor had pledged to build 800,000 homes in the first year, with half of those to be "affordable homes" and give Londoners first dibs to buy them over buy-to-let purchasers or overseas investors.
However, since stepping as Mayor, Mr Khan has accused former Mayor Boris Johnson of "leaving the cupboards bare" where housing was concerned in a damning condemnation of the Uxbridge and South Ruislip MP.
The Mayor had already alluded to lowering expectations on affordable housing during the first Mayor's Question Time in July, where he said he would focus on building "the right kind of homes" rather than a certain number.
In his first Mayor's Question Time, the Mayor discusses the housing crisis
Transport for London land: Boosting or denting city's housing crisis?
The Mayor was also questioned on his plans to secure land from Transport for London (TfL) by having it sold off at a cut price to house builders.
City Hall holds divided opinion as to whether it will help in securing affordable housing, or whether the promise of affordable houses should be kept if it "drives down land value", according to GLA Tories.
Conservative London Assembly member for West Central, Tony Devenish, said: "The more detail I receive on the Mayor’s plans for TfL land the more they concern me.
"He has already confirmed that his 50% affordable housing requirement will cause the land to be sold at lower than market value.
"Now the Mayor has confirmed he will be setting up a joint venture so he can legally proceed with these sales, and that will effectively make Transport for London a large housing developer.
"Phase one will include 75 properties - this will require expertise TfL doesn’t currently have and a large costly back office to support this endeavour."
He added: "London needs to increase it housing stock fast to help lower prices, and this approach will be slow as it’s needlessly complex.
"He should just sell this land, and hold the developers to account with his Mayoral powers, but instead he’s wasting TfL’s already stretched budget on a vanity project."