As his Mayoralty draws to a close, it is already clear that Boris Johnson’s primary legacy will be to have inherited a housing shortage and left a housing crisis. With the typical home now costing near half a million, average private rents at £1,600 and a new definition of affordable housing that is unaffordable to low- and middle-income households, it is easy to see that the cost of housing is causing great hardship for many in this city.

Against this background, it is now reported that for London’s major sites – including opportunity areas and housing zones, where the vast majority of future housing supply in London will be delivered – the Mayor is considering a change of policy to institute a measly 25% target for affordable housing on these sites.

Only Boris Johnson could believe that what London needs right now are more homes that are unaffordable.

This is a parting shot at Londoners who need more affordable housing – a decision that will further compound the bleak consequences of Boris Johnson’s mayoralty. By setting such a scandalously low target on London’s major sites, this latest folly could go so far as to tie the hands of both a future Mayor and local authorities. If introduced, it stands to be as unpopular as it is unfair, whilst its damaging impact will surpass both.

Housing Zones were once seen as an opportunity to boost London’s affordable housing provision. But this target, would water down that opportunity by effectively preventing London boroughs from being able to negotiate up the number of affordable homes on a development beyond the 25% threshold. In fact, this target would be far lower than those currently demanded by many boroughs that have retained the policy Boris Johnson inherited from Ken Livingstone in 2008 that half of all new homes should be affordable.

The ramifications of setting such an unambitious target are obvious. Fewer affordable homes are built, meaning prices for first-time buyers and private rents soar, and the housing crisis becomes ever more deeply entrenched.

Londoners are paying the price for Boris Johnson’s perpetual failure on housing.

Adequate housing provision forms the foundations of any great city, but in London too few homes are being built as it stands. Each year, our capital needs at least 49,000 new homes to accommodate its growing population and the undersupply of housing over recent years. This in turn supports its economy. Boris Johnson’s self-imposed target of 42,000 homes a year already falls far short of London’s need, but in 2014/15 the Mayor built only 18,260 homes.

Insufficient supply has sent house prices and private rents rocketing. First time buyers will find it particularly hard to get a foot on the property ladder with average house prices rising above £500,000 for the first time last year. Average house prices are now £126,000 higher than when the Mayor took office in 2008.

Meanwhile the picture is similarly dreary for the city’s private renters. Boris Johnson has gladly enforced the Coalition Government’s ‘Affordable Rent’ tenure – essentially as a replacement for traditional social rented housing but which defines ‘affordable’ as anything up to 80% of the market rent. In some parts of London, this new tenure is not affordable even to middle-income families.

For some families caught out by the high cost of housing, there is a risk that they will find themselves forced out of the capital. When you also take into account the cost of living in London and the real terms cut to working-age benefits, including tax credits and housing benefit, there’s an increased likelihood of this happening.

Key workers are increasingly being squeezed out of London by unmanageable housing costs. Last November, the then head of the London Ambulance Service said that the organisation is struggling to recruit and retain staff who can no longer afford to live near their place of work.

The Mayor once said that high house prices were “the right problem to have”, but this is a view unlikely to be shared by Londoners, or indeed business leaders. Unaffordable housing costs could stand to have grave consequences for the capital’s economy. A recent CBI and KPMG survey found that 14% of London businesses view the cost of housing as London’s biggest weakness as a place to do business.

Slowly but surely, the cost of housing is undermining London’s economy and public services.

For eight long years Boris Johnson has failed to deliver the homes we need in London. Now it appears he is trying to tie the hands of local authorities and a future Mayor by setting such an unambitious target for affordable housing on our capital’s key development sites.

It’s possible that Boris Johnson is the only person who thinks that it’s acceptable for three-quarters of new homes to be unaffordable. What we need is tougher scrutiny of viability assessments and a push to get as many affordable homes built as possible. Instead the Mayor appears intent on giving developers a free pass in return for a token number of affordable homes.

With the capital’s housing crisis growing by the day, we desperately need homes that ordinary Londoners can actually afford to live in. The very last thing we need is yet more investment opportunities for property speculators and oligarchs at the expense of ordinary Londoners.

Tom Copley AM is Labour’s London Assembly Housing Spokesperson