The SNP government is facing its first political battle in parliament as it makes a move to scrap tolls on the Tay and Forth road bridges.
First Minister Alex Salmond has come under Labour pressure to keep the Edinburgh airport rail link and trams projects on track.
The minority administration is thought to have enough support to get rid of the charges.
It came as the minority government is thought to have gathered enough MSP support to scrap tolls on the Tay and Forth road bridges.
But rivals are ganging up on it to stop the Edinburgh airport rail scheme and trams project being derailed.
During First Minister's Question Time, Mr Salmond said he was not convinced about the Edinburgh transport projects.
The government has expressed concerns about the projects, although final decisions have not been taken.
However, he was urged to continue with them without further delay.
Ministers have doubts that they will offer value for money, but Labour, the Conservatives and the Liberal Democrats have joined forces to demand the projects are not delayed or cancelled without good cause.
Following pressure from Labour, the Liberal Democrats and Tories, the SNP administration will later accept a cross-party parliamentary amendment not to "arbitrarily" delay or cancel the Edinburgh tram and airport rail projects, which have already received parliamentary approval.
'Arbitrary' decisions
Binding decisions
The SNP said residents and businesses in the east of Scotland have been unfairly treated following the removal of tolls on the Erskine Bridge, over the Clyde, and on the Skye Bridge by the previous Scottish Executive.
Scottish Labour leader Jack McConnell demanded of the first minister: "Will you accept that decision and proceed with the new airport rail link with no further prevarication - yes or no?".
A Labour amendment to parliament supported by Tory and Lib Dem MSPs, as well as independent Margo MacDonald, has insisted that toll removal should go hand-in-hand with funding options for a replacement Forth crossing.
Mr Salmond, who expressed concern about "cost over-run" in capital projects, said financial assessments would be brought to parliament at an early opportunity.
It also states that the existing Edinburgh tram and airport rail projects, which have already received parliamentary approval, "should not be arbitrarily delayed or cancelled and that all future major transport project proposals be properly costed, evaluated and prioritised."
He added: "Jack McConnell will know that as part of the perfectly normal constitutional arrangement, except in certain circumstances, the Scottish Executive is not necessarily bound by resolutions or motions passed by the Scottish Parliament."
However, Finance Secretary John Swinney said the government would support the amendment, as it would never decide matters on an "arbitrary basis".
He said: "A decision on the Edinburgh Airport Rail Link and trams can only be taken when the government is in a position to present full financial information to the parliament in a considered fashion. That is the job of government."
