Sadiq Khan's election promise to ensure half of all new homes in London are affordable has been downsized to a "long term strategic target".
Deputy Mayor James Murray admitted to the London Assembly Mr Khan's housing ambitions could take years to achieve.
Mr Khan's election manifesto claimed London needed 50,000 new homes a year.
But Mr Murray refused to set a target until the publication of a fresh strategy document, known as the London Plan.
City Hall said the process involving public consultations and government approval, was expected to take two years.
Mr Murray also conceded the plan to force developers to obey a "50% affordable" target would not happen quickly.
He was challenged by Conservative Andrew Boff to explain the difference between a long term strategic plan and a dream.
It's different because it is a target to which we want to move over the coming years and we want to set out very soon our first steps towards that.