Councils have called for a "total overhaul" of the way bus services are funded.
The Local Government Association (LGA), which represents more than 370 councils in England and Wales, claimed that local authorities are unable to do much more under the current system to support the network.
They issued a report which urged the Government to fund the concessionary fares in full in its spending review, which will conclude in November.
The scheme - which includes free off-peak travel for the elderly and disabled - has had its Government funding cut by 27% over the past five years.
The LGA said this has forced councils to subsidise concessionary fares at the expense of other services such as school transport for over 16s and community support.
LGA transport spokesman, Peter Box, said: "Councils are doing everything they can to support bus users, but are fast reaching a tipping point.
"There are limited opportunities to make further savings. Reductions in funding for concessionary bus fares continue to impact on many services for the elderly, disabled and commuters, for whom bus routes are a lifeline.
"With up to 500 bus routes being reduced, altered or stopped altogether every year, this funding shortfall needs to be addressed urgently."
He added: "What is needed is a total overhaul of the way buses are run and funded as part of the spending review to protect cherished bus services."