The decision follows the rejection of alternative routes for the A82, including a tunnel through the hillside or under the loch, which would have cost some £4 billion.
Transport minister Humza Yousaf will announce today that an 11-mile section of the road will be rebuilt nearly one-fifth wider than the current carriageway, The Scotsman has learned.
The improvement will be between Tarbet, where the A83 branches off the west, and Inverarnan, just north of the head of the loch. The project is expected to cost between £290 million to £380m.
Lorries and coaches often have difficulty passing each other on the tight bends of the road, which is sandwiched between the loch shore and the steep hillside.
Traffic has also increased from 3,500 to 4,000 vehicles a day over the last few years.
No timescale has been given for upgrading the route, which links Glasgow and Fort William, but draft orders to pave the way are planned for next year.
It follows the widening of the road two years ago at Pulpit Rock, midway along the stretch, which removed traffic lights that had controlled a single-track section for 30 years.
Other improvements have included clearing drains and loose rocks from the verge.
Mr Yousaf will say the current 6m-wide carriageway will be increased to 7.3 metres.
He is expected to tell an A82 summit on the future of the road in Crianlarich: “We have looked at the evidence for the A82 and there has been a marked increase in traffic flows, suggesting the need for a wider carriageway on the challenging stretch from Tarbet to Inverarnan.
“I have met many people and businesses who use the route and they have asked for this change to be made.
“Upgrading this part of the trunk road route to a 7.3m carriageway will lead to improved journey times and reliability and will benefit freight carriers.”
However, the IAM RoadSmart motoring group called for overtaking lanes as well.
Neil Greig, its Scotland-based policy and research director, said: “A wider carriageway and along this long section of the A82 is a welcome move which will reduce the stress and danger of driving larger vehicles to and from the Highlands.
“Many car drivers will be disappointed, however, that no safe overtaking opportunities have made it into the final preferred design option.
“Two-plus-one lanes, separated only by white lines, have had mixed success in Scotland but when a physical barrier is present, as in Sweden, they have been very effective. It’s a pity Transport Scotland seems unwilling to at least try some more innovative approaches.”
Mr Greig said the lorry speed limit should also be raised from 40mph to 50mph, after trials on the A9 showed it did not lead to more crashes.
He said: “A widened and upgraded A82 would be a strong candidate for a higher limit to reduce platooning and dangerous overtaking.”