Drivers travelling on the A303 in east Somerset from Mere to Hindon face a 40-mile diversion for 14 weeks when roadworks begin next year.
It is feared there will be major disruption when traffic is diverted via Shepton Mallet and Castle Cary.
The Highways Agency plans to shut a three-mile stretch for reconstruction and resurfacing from the beginning of February 2009.
It said it was not possible to do the work any other way.
Dave Frampton of the Highways Agency said: "We need to reconstruct the road. If we don't do it the road will fail in the next five years or so.
"We've looked at other possibilities such as working at night or having a single lane of traffic flowing through but the fact that we've got deep excavations, we're doing lots of barrier work and getting safety zones for our workforce and the travelling public mean it's just not possible to do it any other way."
London-bound traffic will be diverted off the A303 at the Podimore roundabout near Ilchester and up the A37 to Shepton Mallet, along the A361 to Frome and then on to the A36 near Warminster, rejoining the A303 at Wylye.
Due to a low bridge heavy lorries will all be directed up the A371 through Castle Cary to Shepton Mallet.
Public information days about the closure and diversions are being held in January 2009.
Councillor Henry Hobhouse, Somerset county councillor for the Castle Cary area, described the diversion proposals as "appalling".
He said the amount of traffic generated through the area would be on a par with the Royal Bath and West Show, the only difference being the extra traffic and congestion would be going on for more than three months, rather than four days.
"People in the east of Somerset are going to find life incredibly difficult," he said.
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