The report, Monitoring the mayor’s housing commitments, by the assembly's housing committee, said that 12,555 affordable homes were started in 2017/18, narrowly exceeding the bottom end of the mayor’s annual target of between 12,500 and 16,500 homes.
However, the committee's report said demand for social rent represents almost half of housing need in London, but only 14 per cent of housing starts in Sadiq Khan's mayoral term have been for social rented homes.
According to the committee, 5,355 affordable homes part-funded by the mayor were completed in 2017/18, compared to an average of more than 10,000 a year over the last decade.
But Khan’s office said affordable home starts are at their highest level since funding was devolved to the mayor and that he expects to exceed his target of starting 14,000 affordable homes this year.
Sian Berry, Green Party assembly member and chair of the housing committee, said: "The mayor is letting down Londoners. He promised us more affordable housing but so far has fallen very short of his promises, particularly on social housing, which is our greatest need.
"With young and lower-income people suffering the most from the housing crisis, we can’t wait much longer for his policies to kick in. The mayor must fulfil his pledges. It is in his power to ease the housing crisis and meet his targets – especially as he has received new funding.
"There is now no excuse for genuinely affordable housing to be out of reach for Londoners who want a secure home."
A spokeswoman for the mayor said: "This nonsense report is the exact opposite of the truth. In reality, Sadiq Khan has exceeded all his housing targets and is building a record number of social and affordable homes.
"Last year, City Hall started building more social homes than ever before – more than the rest of England and Wales combined."
In August , Khan called in plans for 1,000 homes in Greenwich after councillors decided to refuse permission, noting the potential to deliver affordable homes.
Later that month, the mayor approved a 441-home scheme in north London after concluding that the project's proposed 50 per cent affordable housing contribution outweighed any negative impacts.