An industry source said the A9 plans would be pushed back from their scheduled completion in 2025 to “nearer 2030”.
They also said the A96 scheme, due to be finished by 2030, could be downsized or “delayed indefinitely”.
The claims came as Transport Scotland admitted it was “taking stock” of the A9 project “to ensure public funds will be spent efficiently and effectively” and “the considerable benefits” are balanced “against financial risk”.
The Scottish Government’s latest infrastructure investment plan (IIP), published on Thursday, showed it plans to spend only £328 million on the scheme between March and 2025-26.
The document described the scheme as “phased dualling”, with no completion date listed.
The previous IIP, published in 2015, stated: “The Scottish Government has given a commitment to complete to dual the A9 between Perth and Inverness by 2025”, describing the date as an “ambitious target”.
A separate Scottish Government document, published in December, said “a range of financing options” would be considered for the A9 project “during subsequent stages of design and assessment”.
These are believed to include involving the private sector.
The project comprises dualling 11 sections of the A9 between Inverness and Perth, but only one has been completed so far, between Kincraig and Dalraddy, south of Aviemore, in 2017.
A second section, between Luncarty and Pass of Birnam, south of Dunkeld, is “anticipated” to be finished in “winter 2021” at a cost of £96m.
The industry source said: “The A9 is likely to be nearer 2030 for completion.
"The Scottish Government is still fully committed, but my understanding is they are looking at potential private finance initiative options for completion.”
On the A96 scheme, between Inverness and Aberdeen, the source said: “It is looking more likely to be be delayed indefinitely, if not downsized.
“It’s possible that only the Nairn to Inverness bit will happen in the near future.”
The latest IIP said only £20m was due to be spent on the A96 scheme between March and 2025-26
It said the £3bn overall cost was an “initial estimate which will be refined and updated as the scheme design for each section becomes more developed”.
The document also stated the delivery timetable was “to be set following completion of relevant statutory procedures”.
The 2015 IIP had restated the commitment to complete it by 2030.
A motoring group said delays to the projects would be a major setback.
Neil Greig, the Scotland-based policy and research director of IAM Road Smart, said: “Any delay to the A9 would be a particularly bitter pill for Highland drivers to swallow.
"The original timescale already promised years of roadworks, but at least promised a safe, high-quality route at the end of it.
"If that promise is now being reneged on, long-suffering A9 users will want to know why.
"Every year the A9 and A96 continue to have killer single-carriageway sections and poor junctions will cost lives as well as stunting the local economy.”
Transport Scotland described the A9 scheme as an “incremental programme”.
Its spokesperson said: “We continue to make significant progress in dualling the A9 between Perth and Inverness, one of the biggest and most complex transport infrastructure projects in Scotland’s history.
“As we approach the conclusion of statutory procedures for the majority of the remaining sections, we are making good progress with the necessary technical and commercial analysis required to shape the most efficient delivery model.
“The design and development process has been protracted by the impacts of Covid-19 and also, quite rightly, ensuring the statutory process concludes with local communities having their chance to input and have any objections resolved appropriately.
“At the same time, the economic climate has changed considerably and unexpectedly.
“For a programme of the scale and significance of A9 dualling, it is simply good, responsible governance to take stock and ensure public funds will be spent efficiently, effectively and balancing the considerable benefits of the programme against financial risk and undue cumulative impacts to the travelling public and local communities.
“We appointed commercial advisors last year and as part of ongoing stakeholder engagement, market consultation will be undertaken this year to inform identification of the most efficient delivery model and programme.
“In such an uncertain economic climate it is essential we discuss our current programme and understand market conditions within the contracting industry to ensure that together we can deliver A9 dualling as efficiently and effectively as possible.”
On the A96 scheme, the spokesperson said: “We are continuing to progress our ambitious plans to dual the A96 between Inverness and Aberdeen, delivering around 88 miles of upgraded road.
“As part of that programme, we are progressing the statutory process on the Inverness to Nairn (including Nairn bypass) scheme, along with the detailed development and assessment of the preferred option on the Hardmuir to Fochabers scheme.
"In addition, we recently completed the route options assessment process and announced a preferred option on the East of Huntly to Aberdeen scheme.
“Delivery of each of the schemes that make up the dualling programme can only commence if they are approved under the relevant statutory procedures and thereafter a timetable for their progress can be set.”
“When completed, the dualling programme will bring many benefits to local communities, businesses, visitors and road users living, travelling and working along the corridor including reduced journey times, improved journey time reliability, improved road safety and opportunities for active travel.”