The SNP's minority administration can ignore votes of parliament, the new First Minister claimed yesterday, as he faced a majority of MSPs demanding that he retains plans for Edinburgh trams and a rail link with the capital's airport.

Alex Salmond quoted Donald Dewar, the first Labour First Minister, as he asserted that he does not have to accept the view of opposition MSPs, who outnumber the SNP's by 81 to 47.

Presiding Officer Alex Fergusson supported the claim, with a ruling that the law makes clear the Scottish Executive must accept Acts of Parliament, and motions of no confidence or about Holyrood's tax-varying power. But ministers cannot be bound by other votes.

The decision is vital to the running of the new SNP government, which may need to be able to ignore votes that go against it, particularly on financial commitments.

The issue was forced into the open yesterday by a row over the future of Edinburgh's trams and airport rail link, after the SNP's election manifesto had argued that both should be cancelled and their combined £1.1bn budgets re-directed to other priorities.

Although the SNP wanted cross-party support for abolition of Forth and Tay bridge tolls, costing the executive up to £15m each year, the debate was widened and sparked a clash over the bigger projects.

Procedural manoeuvring between the parties showed what Holyrood politics could be like for the next four years.

The SNP faced defeat on a Labour amendment, which had LibDem and Tory backing. But the compromises required to win Conservative support were so great that even the SNP was able to support the amendment.

Ministers will make a statement on the Edinburgh projects later this month, with an assessment of their financial viability and prospects of running over budget.

Stewart Stevenson, the Transport Minister, said on Wednesday costs are running out of control, but that has not been backed up with evidence. Mr Salmond pointed to the cost overruns on the new Stirling-Alloa-Kincardine rail link. That was budgeted at £35m and the latest estimate is now between £80m and £85m.

John Swinney, the Cabinet Secretary responsible for finance and transport, said in a subsequent transport debate: "We must be prudent with the public purse when it comes to these projects. I will not sign off projects which are not robust and financially secure."

However, Labour's Wendy Alexander, countered with a warning not to thwart the will of parliament: "If, in the coming weeks, they seek to overturn these projects, the SNP will be sending a signal that they are not about the prudence they claim but about prejudice of geography and petty political interest."

Under attack for cancelling the trams, Mr Salmond put forward the SNP's alternatives, including better bus services and more park-and-ride options. On abolition of bridge tolls, Mr Stevenson said a bill will be published in September, with the tolls expected to go by the end of the year.

More than 500 people are currently working on the Edinburgh tram and rail link project, many of whom would face redundancy should it be abandoned. Tie, the publicly-funded body driving the project for Edinburgh City Council, has around 100 people on the tram scheme alone.

A Tie source said: "There are additional implications for the project's partner organisations, which have around 400 people working on the project.

"The anticipated additional jobs created by the construction phase will also be foregone, including apprentices. The long-term jobs resulting from the operation of the tram will also be lost."

Meanwhile, tendering for shipbuilding contracts has continued under the new administration, despite criticism from the SNP whilst in opposition over granting contracts to foreign yards to build public service vessels.

Calmac, the publicly-owned ferry company, yesterday confirmed that it had sent technical details to seven shipyards, five of which are based outside the UK, to build a £20m ferry to service the Islay route. The shipbuilders, including Ferguson shipyard in Inverclyde, can now bid for the contract to build the ferry, which is due to come into service in 2010.