A main road between Saffron Walden and Haverhill will close for eight weeks in the new year.
The B1054 Hempstead High Street will be shut from January 5 until late February 2022, with no direct access between Radwinter and Steeple Bumpstead while works take place.
Essex Highways will set a 13-mile diversion via Finchingfield.
Affinity Water needs to block the road to replace a water main.
An Affinity Water spokesperson said: "New water pipes are essential to help reduce disruption from leaks and bursts, leave more water in our precious environment, and help to us to provide a reliable, high-quality supply of water to the community long into the future.
"To ensure the safety of commuters and our engineers, a full road closure will be required of the B1054.
"We are sincerely sorry for any inconvenience this work may cause and would like to thank residents for their patience."
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