The total cost of upgrading a six mile (10km) section of the A9 in the Highlands is now expected to run to £308m – £111m more than originally estimated, according to a Scottish government report.

The Tomatin to Moy stretch, south of Inverness, is one of six projects to upgrade of the trunk road from Inverness to Perth to dual carriageway by the end of 2035.

The report on major infrastructure projects said an initial estimated total cost of £197m later rose to £254m and then in July this year to £308m.

SNP Inverness and Nairn MSP Fergus Ewing said a “full explanation” was needed of why the total cost was not mentioned in a recent Scottish government announcement. Transport Scotland has been approached for comment.

Two sections of single carriageway totalling 11 miles (18km) of road have been upgraded over the past decade under the £3bn dualling project.

But 77 miles (124km) of the route has still to be dualled.

When completed, the new Tomatin to Moy section would provide about 20 miles (33km) of continuous dual carriageway between Inverness and Slochd.

The project has been hit by delays, including earlier this month when it was revealed the section was expected to be operational in 2028 and not late 2027 as hoped.

Last year, the construction contract was re-tendered after only one company bid for the work.

In July, the Scottish government agency awarded the contract worth almost £185m to Balfour Beatty Civil Engineering Ltd.

The new report said inflation and costs around a change in contract procurement had led to the increased estimated total cost of £308m.

Mr Ewing said the Scottish government “needed to be candid” with the public on costs.

He told BBC Scotland News: “A full explanation is needed to say why the whole costs were not mentioned in the recent press release.

“The total cost will, I expect, include all preparatory work in route selection, and design, obtaining the necessary legal orders.”

Mr Ewing added: “But until such time as the total business case figures are broken down into components and made public, we remain in the dark.”

Scottish Conservative MSP Jamie Halcro said failure to dual the A9 when promised to had led to the cost “skyrocketing” due to inflationary pressures.

He said: “The nationalists’ broken promises mean taxpayers will need to shell out millions more, not just on the Tomatin-Moy section but doubtless on all other stretches too.”

Expected operational dates of the other remaining sections under the new programme, subject to contract and funding processes, according to the A9 dualling project are:

• Tay Crossing to Ballinluig – by the end of 2028
• Pitlochry to Killiecrankie – by the end of 2030
• Pass of Birnam to Tay Crossing – by the end of 2032
• A9 North, comprising the Crubenmore to Kincraig and Dalraddy to Slochd projects – by end of 2033 at earliest
• A9 Central, comprising the Killiecrankie to Glen Garry, Glen Garry to Dalwhinnie and Dalwhinne to Crubenmore projects – by end of 2035 at earliest

The report also provides updates on other projects.

It said electrification of the Borders rail line and of the Fife Circle rail line had been paused due to an ongoing “refresh” of a decarbonisation action plan.

A secure health unit for young people in Ayrshire, due to be opened by the end of last year, was still not complete and a new National Treatment Centre being added to Forth Valley Hospital had been further delayed due to construction problems.