Glasgow City councillors are worried that Ren- frewshire Council has adopted a 'contributions policy' to raise cash for the transport project.
This would mean developers pay something to the scheme when seeking approval for projects along the route.
There are fears it is insufficient to meet the cost of taking the line from the Southern General out to Braehead Shopping centre.
The funding for the initial phase from the city centre to the SECC and the Southern General has been agreed with Scottish Government funding the project, but not for any extension to Braehead.
A report by Liz Cameron, executive member for development and regeneration at Glasgow City Council said: "Specific funding for the extension of Fastlink into Renfrewshire has not been identified.
"The inclusion of a dedicated development contributions policy in relation to the delivery of the Clyde Fastlink project is welcome in itself.
"Given Glasgow City Council's experience in relation to this policy, however, it is considered that contributions associated with a dedicated development contributions policy would be unlikely to deliver the Fastlink scheme on their own."
Last week the Evening Times reported Glasgow councillors' concern over plans to have Braehead called a town centre for planning purposes amid fears an even greater retail centre would be detrimental to other traditional shopping centres such as Glasgow.
It was argued that Braehead was not a town centre but purely a commercial area.
The report also has concerns over access to Glasgow Airport.
Councillors in Glasgow want their counterparts in Renfrewshire to ensure the Glasgow Airport Rail Link could be reinstated.
The report states: "The council considers that the local development plan should protect the alignment previously identified for construction of GARL, until such time as an alternative route for a surface transport link to Glasgow Airport has been identified and committed for implementation."
A Renfrewshire Council spokesman, said the city council's plans were 'noted', adding: "Renfrewshire, Glasgow and SPT are working together to deliver Fastlink from Glasgow city centre to the Southern General Hospital and on to Renfrew.
"We anticipate the infrastructure in Renfrewshire to be ready for Fastlink in 2016."