Figures in the Local Transport plan (LTP2) show passenger journeys fell from 121 million to 111 million in the last 12 months and Sheffield Green Party leaders today warned if the trend continues bus use will be halved over the next five years.
They want council regulation of fares, routes and frequency in an effort to halt the falling figures.
Green Party transport spokesman Robert Murphy said: "This information is truly shocking. In February council chiefs warned the bus operators they had six months to improve or face regulation. Since then we have seen price rises, service cuts and now the verdict of the passengers themselves. We want to see moves towards regulation immediately to stop this destruction of our once proud and popular bus service.
"It is no exaggeration to say that public transport under this government and council is in crisis. The last Local Transport Plan was a failure and the next just offers more of the same. We believe public transport should be a public service, not a means for producing excessive profits for private operators. It's time to halt the years of decline brought about by deregulation. "
The LTP report blames some of the decline on last year's three-week bus strike, which accounted for four million lost passenger journeys on buses belonging to First Group, the largest bus operator in South Yorkshire.
Mr Murphy said: "Obviously a fair amount was lost then but four million is not even half the total amount lost. That still leaves six million, which shows a downward spiral.
"In London, there is regulation of routes and bus patronage is going up there. However, in most other main cities, passenger levels are going down. The buses are not meeting people's needs and this is putting passengers off."
Last month, bus operators insisted there was no need for Government intervention to give passengers a better deal.
They blamed local councils and the Passenger Transport Executive for the state of bus services, claiming they were not doing enough to ease road congestion to ensure reliability.
Passengers have been deserting buses over the past 20 years.
In the 1970s and early 1980s the county, and Sheffield in particular, had one of the highest rates of bus use in the country, partly due to its famously subsidised fares, which were once as low as 2p a ride.
But Margaret Thatcher's government changed the system in 1986, introducing bus deregulation. That meant any firm could run buses just about anywhere they wanted.
People turned instead to the car – leading to traffic-choked roads – and bus companies complain it is congestion on the county's roads that makes their services less reliable and stops people from using them.
They want local councils to give buses priority at traffic lights and improve the road system to make it easier for buses at peak times.
The LTP is made up of partners from Barnsley, Doncaster, Rotherham and Sheffield councils and the South Yorkshire Passenger Transport Executive and Authority.
First Commercial Director Brandon Jones said: "Regulation would carry significant public subsidy and does not provide any solutions to improving traffic congestion or creating consistent journey times.
"By working with our partners more effectively we can tackle the main concern of customers, which is the provision of consistent journey times.
"According to the South Yorkshire Passenger Transport Executive the reliability of First services is improving and now stands at 93 per cent.
"Our customers have also seen improvements on services where we have added extra buses to maintain reliability."