The new Scottish Government's first piece of legislation ran into trouble on its first outing yesterday, as opposition MSPs challenged the lack of planning behind the removal of Forth and Tay bridge tolls.

St Andrew's House officials were forced into defending why there has been no environment impact assessment from a projected sharp increase in traffic levels.

They claimed there was no need for public consultation, which precedes other bills, because the measure had been in the Scottish National Party manifesto. A senior official said the over-riding priority was to address the "social justice" issue of Fife motorists paying Scotland's only remaining tolls.

At a hearing of Holyrood's Transport, Infrastructure and Climate Change Committee, it emerged that the Scottish Government wants the Forth Estuary Transport Agency (Feta), which owns and runs the Forth Road Bridge, to retain the borrowing powers it has, even though it will lose any income stream from tolls with which to pay off debt.

That raises the possibility that Feta could borrow from private sources the full cost of a new Forth crossing, priced at more than £1.5bn, and could have that repaid over decades through regular payments by the Scottish Government. However, such backdoor borrowing powers are likely to find opposition from the UK Treasury, which is against the Holyrood administration borrowing in its own right.

Although the Greens are the only party at Holyrood opposing the removal of tolls, a major problem for the legislation is the official estimate by Feta that the change will increase traffic by as much as 21%, when it already runs at 37,000 vehicles a day - 7000 higher than the maximum level to avoid congestion. Erskine bridge traffic is up 23% since tolls ended.

Stewart Stevenson, the Infrastructure Minister told the committee the government wants to ensure traffic levels over the Forth do not rise above 2006 levels.

Labour MSP Des McNulty said it was not plausible to argue there is no need for a study of environmental impact from increased traffic.

He warned that if traffic increases the load on the Forth Road Bridge its life could be shortened, and heavy good vehicles may be barred in the next few years, with "devastating impact" for the Fife and Tayside economy.