He says that latest figures from Metropolitan Police show that there are now only 3,253 specials in the capital, 43% fewer than in May 2012 when he made the pledge, with numbers in Harrow falling 47% from 138 in May 2012 to only 73 in December 2015.
Special constables are volunteer police officers with all the powers of police officers.
Navin Shah says the falling number of special constables comes on top of significant cuts to the number of police community support officers (PCSOs) over the past few years.
Met statistics reportedly show that in Harrow since 2010, 110 PCSOs and 403 Police officers have been cut from the streets, 15% of the original number.
'Dramatic and continuing fall'
Labour London Assembly Member for Brent and Harrow Navin Shah, warned that the cumulative impact of the drop in Specials, PCSOs and police officers risks leaving a “much reduced police force for his successor.”
Navin Shah AM said: “In 2012 Boris recognised the significant role Special Constables play in providing an important link between the police and public and bolstering our police service.
“Despite his promise to double their ranks what we’ve actually seen is a dramatic and continuing fall in the number of Special Constables, including in Harrow which has seen a 47% decline.
“At this rate he may well have less than half of what he started with by the time he leaves office.
“With PCSO and police officer numbers also in decline, the falling number of Specials should be of real concern.
"Boris should be asking why the Met is failing to attract enough people, is it because the opportunities are not well enough advertised or is the increasing pressure facing the police putting people off?
“Between the drop in Special and PCSO numbers, Boris looks set to leave a much reduced police force for his successor.”
'Recruited 5,000 officers in last 3 years'
In response to the figures, Stephen Greenhalgh, Deputy Mayor for Policing And Crime, said: "Frontline policing is a top priority for the Mayor and at a time when police numbers nationally have fallen, he has protected numbers in London at around 32,000 and put an additional 2,600 officers into neighbourhoods since 2012.
"In the face of significant financial pressures, the percentage of officers in visible frontline roles has increased substantially and the Mayor has maintained his commitment to of a minimum of one dedicated PC and one dedicated PCSO per ward.
"The Met has recruited 5000 officers in the last 3 years and many of these were special constables who have now moved from their volunteer roles into permanent jobs in the regular police force.
"The Met are actively seeking to recruit more specials and I would urge Londoners to apply to the variety of important and hugely rewarding voluntary roles on offer."