The £3 billion project - one of biggest in Scottish history - is likely to mean continuous roadworks for a decade, starting in ten days’ time.
Infrastructure secretary Keith Brown launched work yesterday on the first five-mile section, between Kincraig and Dalraddy, near Aviemore.
He described it as the Scottish Government’s “flagship infrastructure investment project”, which will cost more than twice as much as the Queensferry Crossing that has already put a strain on official coffers.
The Transport Scotland agency, which is in charge of the project, said that how the 80-mile scheme would be funded had still to be decided. A spokesman confirmed funding had been committed so far for only the £35 million Kincraig stretch, which is due to be finished in summer 2017.
He said it had yet to be agreed how the other 11 sections would be funded, which could include traditional government spending or a private-sector financing method, which is being used for the £745m Aberdeen bypass.
That could spread the cost of the work beyond the planned completion date of 2025.
The SNP pledged to dual the single-carriageway sections of the road after coming to power in 2007. However, only about two miles have been completed so far, at Crubenmore, near Dalwhinnie, in 2011 at a cost of £10.5m.
Mr Brown has come under pressure to speed up the work, and has said half the scheme will be complete by 2022.
But finishing it on time could prove a major challenge - even if full funding is secured.
This is because the widened A9 will have to be built within a narrow corridor around the road, some of it through challenging terrain. Designs are still to be completed.
Mr Brown said: “It is no understatement to say we are marking an important milestone in what will be one of the largest and most challenging infrastructure projects in Scotland’s history.
“We are at the start of a long journey to deliver the entire programme.
“But we are committed to this programme and we will remain focused to ensure this work, which is now at the hard end of construction, is seen through to a successful conclusion.”
The Kincraig contract, won by Wills Bros Civil Engineering and John Paul Construction, will involve 700,000 tonnes of excavation and the equivalent of 25 football pitches of new road.
The speed limit will be reduced from 60mph to 40mph from Monday, 21 September, enforced by average speed cameras.
Institute of Advanced Motorists policy director Neil Greig said of the work: “This is good news for users of the road, but there will be ten years’ of roadwork pain before we get the complete gain of a safer Perth to Inverness route.
“Our hope is any savings made on Scottish Government projects will be ploughed into earlier completion.”