Nearly half of bus routes in England could be scrapped due to a lack of funding, councils have claimed.
Analysis for the Local Government Association (LGA) found that the free bus pass scheme was underfunded by an estimated £652 million in 2017/18.
Councils say they are being forced to fill the gap between this government funding and what the scheme costs.
Free bus passes for off-peak travel are a legal entitlement for people aged over 65, or those with a disability.
But budgetary constraints mean councils are spending less on discretionary items such as free peak travel, post-school transport and supported rural services.
Nearly half of all bus routes in England receive partial or complete subsidies from councils.
The LGA warned that these services are at risk as local authorities will struggle to maintain current levels of support unless they are given more funding.
It wants the Government to reinstate the full funding of the costs of the national concessionary travel scheme.
LGA transport spokesman Martin Tett said: “An estimated funding gap of £652 million a year for concessionary travel is unsustainable for councils already struggling to protect other subsidised bus travel in rural areas, or helping young people with their travel costs.
“Properly funding the national free bus pass scheme is essential if the Government wants councils to be able to maintain our essential bus services, reduce congestion and protect vital routes.