Sadiq Khan was sworn in as Mayor of London at Southwark Cathedral on 7 May 2016IBTimes UK

The former Conservative mayor of London Boris Johnson has been accused by his successor at City Hall of having "grossly let down Londoners by leaving the cupboard bare when it comes to delivering affordable housing". Housing is the single most important issue to Londoners, according to polling, as a shortage sends rents and house prices spiralling.

Sadiq Khan, who won the mayoral vote for Labour on 5 May 2016, said he had demanded an "urgent audit" when he took office of how City Hall was tackling the housing crisis. He accused Johnson of overseeing the lowest level of new affordable housing since records began in 1991, with just 13% of the units being given planning permissions in the city classed as affordable.

"I am determined to fix London's housing crisis and ensure that all Londoners have the opportunity to rent or buy a decent home at a price they can afford, but the scale of the challenge is now clearer than ever and we're not going to be able to turn things around overnight," said Khan, who also accused the mayor of falling far short on promises to free up more public land for housing and grow the number of construction apprentices to tackle the industry's crippling skills shortage.

"Disappointingly the new mayor isn't coming clean on his housing numbers — he fails to mention 100,000 affordable homes were completed between 2008/2016 — a record," said a spokesman for Johnson, who is MP for Uxbridge. "He also knows that last year's completions were low for a reason - the start of a new government investment round - yet he fails to mention that the previous year saw a record 18,000 affordable homes completed - more than any point since 1981.

"It's concerning that the new mayor trumpets an audit and talks of 'the shocking scale of the challenge' when he well knows that this data is already public and was published on the GLA website under the last mayor.

"Mr Khan also knows that the GLA released all of its surplus land under Boris Johnson — that's 414 hectares which will deliver about 50,000 homes — a legacy which will greatly benefit this mayor's housing numbers, if only he'd bothered to check."

Housebuilding in London is rising sharply, but still around half the level needed to meet demand, estimated by City Hall to be around 50,000 new units a year. In 2015, there were 24,230 new housing starts, a 34% annual rise and the highest level since the early 1990s.

Between 2005 and 2015, the average London house price increased to 90% to £536,000, according to the Office for National Statistics (ONS). Over the same period, the private rental index from the ONS increased by 35% in London. To contrast with pay, average weekly earnings between 2005 and 2015 rose just 19% in London.

Khan won the election on a ticket pledging to ensure that half of all new homes built in London would be affordable, a policy his critics and some in the construction industry say is unachievable.

"We will be outlining our plans in the coming months, but one of the first things we can do is work with Transport for London to fast-track their numerous surplus sites for development that have previously just been sat on," Khan said. "There is no doubt we have our work cut out, but I plan to personally get to grips with the mess that has been left behind and will insist on far higher levels of affordable housing in new developments."