According to the figures, a total of 8,114 affordable homes were completed in 2012/13, against a target of 7,562 completions for the financial year.
But the figure is half the number of affordable homes completed in the capital through the affordable housing programme in 2011/12, according to figures published separately by England's housing and regeneration quango the Homes & Communities Agency last year. According to the HCA's figures, there were 16,176 afforable housing completions in London in the 2011/12 financial year.
The mayor’s office said in a statement that the figures showed that Boris Johnson is "now on track" to deliver 55,000 affordable homes between 2011 and 2015. "At the halfway point he has already completed over 25,000 affordable homes," the statement said.
London mayor Boris Johnson said: "Increasing the supply of affordable homes in London is intrinsically linked to the future economic growth of the capital. Without homes to live in the hard-working Londoners who keep this city thriving will be forced to move elsewhere taking with them their energy and drive.
"While there is a growing momentum behind affordable housing delivery, we need to redouble our efforts, which is why I will continue to call on Government to give London all the tools and funding it needs to increase the supply of affordable homes in the capital."
But Labour London Assembly planning spokeswoman Nicky Gavron said: "The mayor’s poor performance on affordable housing starts is bad enough, but his completions are even more worrying. His completions are down over a half from what they were the previous year, from 17,240 to 8,114.
"It’s no wonder housing minister Mark Prisk wrote a letter to Boris, leaked early last month, telling him to get a move on and start a real building programme to deal with the capital’s housing crisis."
The Greater London Authority said that the full statistics for 2012/13, including a detailed breakdown of all affordable housing starts and completions by London borough, will be published in early summer.
NOTE: This story was this article was amended at 3:45pm on Wednesday 3 April to add a comment from the London Labour Party which had not been able to provide a statement by the time of initial publication.