The leader of Edinburgh Council has been urged to not spend "a single penny" of taxpayers' cash on a new tram line if the project is approved.
Plans to extend the city's line south are set to be unveiled in the current council term and it is estimated this could cost around £1 billion.
The Scottish Government has backed an expansion of the tram network which it said would provide "improved connectivity between Edinburgh and the surrounding communities".
READ MORE: Edinburgh tram line extension could see it run from Granton to Dalkeith
Early blueprints published by the council have also set out hopes for the trams to run from Granton to Dalkeith via the Royal Infirmary in future, with possible extensions to Sherrifhall park and ride and Newcraighall.
However, councillors have sought assurances on how the next phase would be funded if a business case is supported.
Kevin Lang, Liberal Democrat group leader, called on city leader Cammy Day to commit to not using any council tax income for the project at a City Chambers meeting on Thursday (December 15).
He said: "One of the big decisions which we will need to make in this term of the council is whether to proceed with further extensions to Edinburgh Trams.
"There will be big discussions to be had about routes, location, timetables, funding models."
Councillor Lang highlighted the authority's finances are "extremely stretched".
He added: "Can I ask the council leader if he is able to give a guarantee that not a single penny of council tax money in Edinburgh will be used to pay for any tram extension?"
Responding, Cllr Day, Labour, agreed that there should "not be council tax money spent on this".
And he said the business plan for an extension would "commit to use fares from trams" to pay for the line, adding he hoped support would also come from Transport Scotland.
"I think money for a capital infrastructure project of this level needs to come from Transport Scotland and the governments that fund this council," Cllr Day said.
He said the success of Edinburgh Trams "is proven" and added councillors should "at least" debate extending the line south out to the bioQuarter, Royal Infirmary and "many thousands of new homes".
"Is it absolutely something we need to have an honest and frank discussion about," he said.
The botched construction of the first line from the airport to York Place - completed £400m over budget five years late - was mostly financed by the Scottish Government with some council cash used.
Meanwhile the extension to Newhaven, which is set to open in the spring, has been funded with borrowing to be paid back by future tram fare revenues.
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