The admission came as the mayor launched his first draft housing strategy yesterday. It follows claims that his manifesto pledge of 50,000 affordable homes by 2011 will not be fulfilled without a radical rethink.
Johnson responded to Green Party calls to clarify how many homes will be new-build after more recent statements that the target is a gross figure including second-hand acquisitions.
The mayor said he expects around three-quarters to be newly built. He has set negotiable affordable home targets for each London borough, which will be finalised early next year (Planning, 31 October, p1).
The London Assembly planning and housing committee advised Johnson to harness £4 billion of affordable housing investment in a briefing on the effects of the credit crunch.
Committee chairwoman Nicky Gavron said: "The mayor must focus on a mix of social rented and an increased amount of intermediate rented housing, with an option to buy as market conditions change."
The London Borough of Southwark this week reduced the affordable quota for the Larnaca Works project from 35 to 25 per cent after the developer said it would otherwise be unviable.