Increases in workloads have made it impossible for some of the officers in England and Wales to volunteer alongside their day jobs, claims the Police Federation.
The Association of Special Constabulary Officers has described a significant fall in numbers across the two nations as a "huge loss" to policing.
Home Office data shows South Yorkshire Police had 137 special constables in March this year – up from 126 the year before.
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However, it represents a (49 per cent) drop compared to 2011, when there were 269.
The officers, also known as "specials", hold the same powers as police constables and work a minimum of 16 hours a month as volunteers.
A fall in the number of specials within South Yorkshire Police over the decade came alongside a 5% decrease in full-time police officers, despite a Government-backed recruitment campaign for 20,000 more officers nationally by 2023.
Across England and Wales, the number of special officers has reduced by more than half over the past decade, from 18,421 in 2011 to 9,174 this year.
During the period the number peaked at 20,343 in 2012 – following the end of a three-year national recruitment programme – but has since fallen year-on-year.
The Police Federation for England and Wales said a recent focus on recruiting more paid police officers, including some former specials, and an increase in workload for the volunteer officers were behind the demise in numbers.
Chairman John Apter said: “More and more has been expected of special constables.
“These extra pressures have caused some to leave the service, as they cannot juggle their day jobs with what is expected of them.
“We need their support, and we need more of them.”
The ASCO has called for a national recruitment campaign for more specials, claiming they provide "enormous value" to community policing, as shown during the Covid-19 pandemic.
The Home Office figures show the equivalent of 429 full-time police officer roles were filled by former special constables across England and Wales in 2020-21.
Of those, three were in South Yorkshire Police.
The National Police Chiefs' Council's five-year Special Constabulary National Strategy said specials should be used to provide support to forces coping with ever-increasing demand.