A Highland MSP is concerned that the cost of improvement work on a section of the A9 in the far north could rocket due to landfill tax reforms.
Rhoda Grant is seeking confirmation from the Scottish Government that the project planned for Berriedale Braes in Caithness would be exempt from paying the levy.
From April 1, a body called Revenue Scotland, instead of HM Revenue and Customs, will collect landfill tax.
Draft road orders for the improvement scheme, expected to cost at least £2.5milion, were published by the Scottish Government in November.
The A9 drops from 492ft to just 65ft as it enters a steep-sided valley at Berriedale and drivers have to contend with 13% gradients and a tight hairpin bend at the bottom of the slope.
Mrs Grant, a Labour MSP, said: “The Berriedale Braes present a major transport infrastructure project that already has a projected cost of around at least £2.5million.
“The steep gradient on the section that is planned for improvement means that significant material will have to be excavated and disposed from the site, described by Transport Scotland as a ‘major part’ of this work.
“Although it looks as though the excess material could be used in the project or used elsewhere for filing, I want a cast-iron commitment from the government that there will be no more delays to this project and no contractor will be hit by massive landfill tax charges.”
A spokeswoman for SNP ministers said roads agency Transport Scotland was required to comply with landfill tax legislation for all construction projects.
“Current indications are that the surplus soil and rock from Berriedale may be exempt as it could be used for filling a disused quarry,” she added.
“However, before we are able to confirm exemption, checks are required to ensure the quarry has the requisite planning consent and that the material being deposited is acceptable.”
The spokeswoman said the government was making good progress on the new design of the road and orders, assuming any objections were withdrawn, would be laid in the spring and contract documents would be drawn up in the autumn.