Drivers accept travelling a mile or two out of their way when diversions are put in place for roadworks.

But motorists on the A303 in east Somerset will face a 40-mile diversion from Mere to Hindon for 14 weeks 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.

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."

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 onto the A36 near Warminster, rejoining the A303 at Wylye.

Due to a low bridge, lorries will be directed up the A371 through Castle Cary to Shepton Mallet.

Public information days about the closure will be held in January 2009.

Cllr Henry Hobhouse, Somerset county councillor for the Castle Cary area, described the diversion proposals as "appalling".

He said traffic would be on a par with the Royal Bath and West Show every day.