Sadiq Khan today downplayed his chances of meeting his flagship pledge to ensure that half of all new homes built in London will be affordable if he becomes mayor.
Labour's mayoral candidate said he may "fall short" of his 50% affordable target on all new homes.
"I'd rather have a target at 50% and fall short than not have a target and do nothing at all," he told Politics.co.uk.
Khan has promised to reintroduce the target, which was controversially withdrawn by the current mayor Boris Johnson back in 2008.
At the time Johnson pointed out that his predecessor Ken Livingstone had failed to meet his own 50% target in any of the years he was mayor.
Khan admitted that, like Livingstone, it would be "tough" for him to meet the pledge, particularly in the earlier part of his time in City Hall.
"It's going to be tough for the first few years for a number of reasons: getting the land organised, and finding the finance in [my new housing agency] Homes for Londoners..."
"I can't pull a lever on May 6th and everything becomes hunky dory overnight, but we've got to be ambitious, otherwise things will just get even worse than they are now."
He insisted that, unlike in previous administrations, there was a growing "willingness" from business and local government to dramatically increase housing supply.
"We've got to start turning this tanker around, otherwise London is going to be even more hollowed out than it currently is," he told Politics.co.uk.
"If we had this conversation four years ago before the last mayoral election, we would have been talking about nurses, porters and bus drivers not being able to live in London. Now it's chief executives, junior doctors and senior journalists not being able to live in London and unless we address that dramatically, things will go from bad to worse."
Khan's comments come as a new poll finds him extending his lead over his Conservative rival Zac Goldsmith. The Comres poll, for ITV and LBC, found that Khan is now ten points ahead on 55% to 45%, once second preferences are taken into account.
The poll also found that Tory attacks on Khan for being "radical" and associated with extremists are failing to have a big impact. Just 14% of Londoners said they would describe Khan as "extreme" compared to 10% who said the same of Goldsmith. The poll also found that Khan is narrowly more trusted to keep London safe, by 26% to Goldsmith's 25%.
Just 14% of Londoners say Sadiq Khan is "extreme" compared to 10% for Zac Goldsmith (Comres/ITV) pic.twitter.com/1vEsss4LFo— Adam Bienkov (@AdamBienkov) April 5, 2016