A £2billion deal to create a "super highway" connecting London and the Devon area is to be announced by the Government today.
The Government is set to reveal details of its plan to transform the A303 in a bid to end road misery for its long-suffering users.
Ministers are to sanction seven schemes to dual the stop-start single carriageways from the M3 at Basingstoke to the M5 at Taunton,
The plan, much-anticipated but detailed for the first time today, involves digging a tunnel of at least 1.8 miles long under Stonehenge.
Costing up to £1.3billion to get under the sacred stones, the Government believes its proposal will get support from heritage organisations that have thwarted previous attempts to tackle the traffic blackspot.
The deal avoids bulldozing a new dual carriageway through the Blackdown Hills in Somerset, an area of outstanding natural beauty, where opposition has been another brake on improving the trunk road.
Instead, the A358, which forks north off the A303, running from Southfields to Taunton at the M5, will be turned into a dual carriageway.
This allows the Government to make the claim that drivers taking the A303 will travel on a dual carriageway from London to within 15 miles of Land's End – providing a genuine alternative to the M4.
On the Blackdown Hills, "smaller-scale improvements", such as passing roads, will be built on the A303-A30 section between Southfields in Somerset and Honiton in Devon, as big improvements would be "challenging".
The three-mile single-carriageway section between Sparkford and Ilchester will also be dualled.
Prime Minister David Cameron said: "Upgrading the A303 is a crucial part of our long term economic plan for the Westcountry."
In a package that ministers will say will create 1,300 jobs, the Government today details four initial schemes:
* In Wiltshire, two schemes will deal with problems around Stonehenge.
First, constructing a bored tunnel of at least 1.8 miles long as the road passes Stonehenge. Second, a bypass of Winterbourne Stoke to link the existing dual carriageway section around Amesbury with the dual carriageway section at Berwick Down.
English Heritage and National Trust have supported the proposals and their assessments have found that a twin-bored tunnel of somewhere between 1.6 and 1.8 miles long, if designed well, could have a beneficial impact on the World Heritage Site.
The estimated cost for the proposal is between £860 million and £1.3billion.
* In Somerset, dualling the three-mile single-carriageway section between Sparkford and Ilchester on the A303.
* In Somerset, turn the A358 between Southfields and Taunton in to a dual carriageway.
* On the Devon-Somerset border, funding has been set aside for smaller-scale improvements to the A303-A30 section between Southfields and Honiton to improve safety and journey times.
More details of the time-scale – such as when over this and the next parliament work will begin and be completed – is likely to be revealed this week.
The announcement on the A303 is part of what ministers will hail the "biggest upgrade to roads in a generation", arguing better transport links are vital to economic growth in the regions.
The total package nationally will amount to £15 billion, and will involve more than 100 new road schemes.
Mr Cameron said: "Building a tunnel to alleviate congestion around Stonehenge and dualling further stretches of this vital transport link have been talked about for many years. Only under this Government is this essential work finally going ahead.
"Improving the A303 will help the local economy and build on the extra 121,000 jobs that have been created since 2010, ensuring more people in the Westcountry have the financial security of a regular pay packet for them and their family.
"And it is worth pointing out that spending hundreds of millions of pounds on roads like this is only possible because of the difficult decisions we have taken elsewhere as part of our long term economic plan."
Danny Alexander, Liberal Democrat Chief Secretary to the Treasury, overseeing the road-building schemes, said: "The dramatic upgrade of the A303 near Stonehenge will deliver the best for the future, whilst preserving the best from the past.
"We'll use 21st century tunnelling technology to help make this road the modern super highway that the South West needs to reach its full potential, while better protecting one of the UK most important national monuments."
The announcement represents a victory for South West councils and businesses who have campaigned for an upgrade, which they say has stunted economic growth.
One independent study suggested an upgrade could be bring £41billion into the economy, create more than 21,000 jobs. Tourism alone could expect a £12billion windfall in the South West.
