SNP ministers were warned more than 15 years ago that the A9 between Perth and Inverness might never be dualled completely, official files have revealed.

Newly released Scottish cabinet papers show then finance secretary John Swinney shared doubts about the project with Alex Salmond and other senior colleagues in August 2008.

He said problems acquiring land and engineering obstacles could prevent it ever happening.

The SNP would later pledge to dual the road by 2025, a promise it broke spectacularly.

Last month the completion date was revised to 2035 at the earliest and the scheme costed at £3.7bn, leading to claims the government hid the scale of delays.

The SNP manifesto of 2007 committed the nationalist government to publishing a 10-year plan to “transform Scottish road safety” and said the “dualling of key roads, such as the A9, would contribute to fewer lives being lost”.

The road, a key transport artery to the Highlands, is notorious as an accident blackspot, with many fatalities occurring where it switches between single and dual carriageway.

The minutes of the Scottish cabinet of 5 August 2008, which was held in Inverness, include an item titled “Improving Road Safety on the A9”.

They state: “Mr Swinney said that there was continuing concern over the number of fatal accidents on stretches of the A9 from Perth to Inverness. 

“He said that the Strategic Transport Review Project (STRP), which would be presented to Cabinet in October, would outline plans to make significant safety improvements to the A9, amongst other things. 

“He said that ten road improvement works were currently underway or being planned, to improve sections of the A9. 

“Some of these would create additional stretches of dual carriageway and others would create overtaking lanes on one side of the carriageway.” 

However the finance secretary said dualling the full 80 miles of the A9 might be impossible.

“He said that, while dualling of the A9 remained the overarching policy intention, this might not be possible in some places, due to difficulties with land purchase or significant engineering concerns,” minutes released by National Records of Scotland state.

Despite the misgivings, the SNP’s 2011 manifesto for Holyrood said it was “fully committed to dualling the A9” and in November 2011 ministers announced the 2025 completion date.